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PCM20210805_SICP_1.2.6_Example_Testing_for_Primality_I
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### A Pluto.jl notebook ###
# v0.20.4
using Markdown
using InteractiveUtils
# ╔═╡ b58f3d47-bc97-4b54-ad3e-19b775e6849c
begin
using Pluto
using Plots
using LaTeXStrings
using GraphRecipes
using Primes
#----------------------------------------------------------------
println("pkgversion(Pluto) = ", pkgversion(Pluto))
println("pkgversion(Plots) = ", pkgversion(Plots))
println("pkgversion(LaTeXStrings) = ", pkgversion(LaTeXStrings))
println("pkgversion(GraphRecipes) = ", pkgversion(GraphRecipes))
println("pkgversion(Primes) = ", pkgversion(Primes))
end # begin
# ╔═╡ fcaf83d0-f611-11eb-1bc9-31384cf290cb
md"
==================================================================================
#### SICP: [1.2.6 Example: Testing for Primality I](https://mitp-content-server.mit.edu/books/content/sectbyfn/books_pres_0/6515/sicp.zip/full-text/book/book.html)
##### file: PCM20210805_SICP\_1.2.6\_Example\_Testing\_for\_Primality\_I
##### Julia/Pluto.jl-code (1.11.3/0.20.4) by PCM *** 2025/02/04 ***
==================================================================================
"
# ╔═╡ 6d09aba9-b862-4e38-a4a9-fd718d4744c4
md"
##### 0. Introduction
*Since ancient times, mathematicians have been fascinated by problems concerning prime numbers, and many people have worked on the problem of determining ways to test if numbers are prime.*(SICP, 1996, 2016)
"
# ╔═╡ 23534646-17d3-4cba-8505-10991158a47c
md"
---
##### 1. Topics
- *exponentiation* with ^
- *case* analysis by $if$ <condition> <epression> $elseif$ <condition> <epression> $else$ <epression> $end$
- function-wide *scope*
- *jumps* by $break$
- *Fermat*'s Little Theorem
- *coprimes*
- *package* $Primes$
"
# ╔═╡ b64b7617-c882-44b3-9400-863138f3678e
md"
---
##### 2. Libraries
"
# ╔═╡ 35a00708-a1ff-4b0c-a4eb-50452e394e06
md"
---
##### 3. SICP-Scheme-like *functional* Julia
###### 3.1 *Generic* (*Polymorphic*) function $isprime$: Searching for *Divisors* (= *Factors*)
*One way to test if a number is prime is to find the number's divisors. The following program finds the smallest integral divisor (greater than $1$) of a given number $n$. It does this in a straightforward way, by testing $n$ for divisibility by successive integers starting with* $2$.(SICP, 1996, 2016)
"
# ╔═╡ fc96aecc-a50e-4b17-aeda-1294bdf2ec0b
md"
---
###### Requirement: function $square$
"
# ╔═╡ 66a32388-59bf-4cda-946d-a299487c8760
^(3, 3) # prefix exponential operator '^'; ==> 27 --> :)
# ╔═╡ da94c3aa-381d-4604-bdd8-298f74ccd998
square(x) = ^(x, 2) # definition of square
# ╔═╡ 6dce0f0d-290f-429b-8c4d-d9a3e9ea5357
square(11), typeof(square(11)) # for Int64
# ╔═╡ 22f5327d-b4ea-4eae-badb-cf8a139b4bd3
square(11.), typeof(square(11.)) # for Float64
# ╔═╡ d123c9b7-7473-40b3-981c-03a4c88e2126
md"
---
###### Requirement: function $remainder$ ...
... *from Euclidean division, returning a value of the same sign as $x$, and smaller in magnitude than* $y$. (see Julia-doc)
"
# ╔═╡ 414fada3-20bd-4085-9dcc-ae279600ad30
remainder = rem
# ╔═╡ 038c0c86-ec82-4f7d-a271-a06490995528
%(3, 9), rem(3, 9), remainder(3, 9)
# ╔═╡ 3107665f-2e0e-493e-aec8-e536ccea0492
%(9, 3), rem(9, 3), remainder(9, 3)
# ╔═╡ fc40132c-b9b4-4c04-a67a-88f382482189
%(13, 9), rem(13, 9), remainder(13, 9)
# ╔═╡ c14293eb-0a71-4e59-8a85-d65354ca1db0
%(9, 13), rem(9, 13), remainder(9, 13)
# ╔═╡ 9b30ad81-9829-48f6-b125-9a61ee3bcc05
%(13.0, 9.0), rem(13.0, 9.0), remainder(13.0, 9.0)
# ╔═╡ 4c9de0d0-dd9d-4371-9a1b-9523d641ad32
%(9.0, 13.0), rem(9.0, 13.0), remainder(9.0, 13.0)
# ╔═╡ d50c7a2f-a7cf-4b1a-8b07-c3cdf96024d0
md"
---
###### Requirement: function $divides$ ...
"
# ╔═╡ 452f1c9a-d2b5-46bf-adcc-af9f80b62d68
divides(a, b) = ==(remainder(b, a), 0)
# ╔═╡ 19cf97ec-4c9a-4069-9dce-fa265bb904e5
divides(3, 9)
# ╔═╡ d56f43ca-11dc-491f-b6ad-f332a2e2731c
divides(3, 10)
# ╔═╡ c37fb2db-62ef-4eeb-a7cf-27a3cde6bb64
md"
---
###### Requirement: function $smallest\text{-}divisor$
(SICP, 1996; 2016, p.65f)
"
# ╔═╡ e78e17f2-c9fa-4f5b-aefc-8c3e580f117f
function find_divisor(n, test_divisor)
>(square(test_divisor), n) ? n : # stop condition
divides(test_divisor, n) ? test_divisor :
find_divisor(n, +(test_divisor, 1)) # tail recursive call
end # function find_divisor
# ╔═╡ afcd5e13-b61a-491a-bf4f-47ea59fe112d
smallest_divisor(n) = find_divisor(n, 2)
# ╔═╡ c1fd79b5-abfa-44b6-bc10-783f4b1c9924
smallest_divisor(1) # ==> 1 --> :)
# ╔═╡ f1aa9700-1c4a-4a7d-8563-595984bd4a19
smallest_divisor(7) # ==> 7 --> :), because 7 is prime
# ╔═╡ 5504eb77-0e64-4321-8db9-6449deaa401f
smallest_divisor(11) # ==> 11 --> :), because 11 is prime
# ╔═╡ b8e74e16-deed-4903-aaea-d68e3e9d6024
smallest_divisor(15) # ==> 3 --> :)
# ╔═╡ 46cc1617-f160-4041-bd79-193d311afb61
smallest_divisor(97) # ==> 97 --> :), because 97 is prime
# ╔═╡ db02e8a1-4131-4c0f-9975-6223dafd1631
md"
---
###### *Generic* (*Polymorphic*) function $isprime$
(SICP, 1996; 2016, p.66)
The [first 25 primes](https://en.wikipedia.org/wiki/Prime_number) are: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97
"
# ╔═╡ 768cc486-5e6e-4ce1-bf2c-a35151a4793f
myIsprime(n) = ==(n, smallest_divisor(n))
# ╔═╡ dac9cfc1-6b2d-4458-82a6-cf9e2c79fcc1
myIsprime(1) # ==> true --> :); some say 1 is not a prime
# ╔═╡ b5b5a466-e78e-49c0-8ee9-765644596e27
myIsprime(2) # ==> true --> :); 2 is prime
# ╔═╡ 56beed19-5ee7-458b-bd50-3afb9e38e342
myIsprime(3) # ==> true --> :); 3 is prime
# ╔═╡ a344aaaa-af28-4345-8e96-a6b735fb6023
myIsprime(4) # ==> false --> :)
# ╔═╡ eb89783e-e948-42d8-b7d5-857a719da9b4
myIsprime(5) # ==> true --> :); 5 is prime
# ╔═╡ 2754573a-24ea-4a7b-85c3-303004aec2e2
myIsprime(6) # ==> false --> :)
# ╔═╡ 2a3be054-6361-4b44-9ceb-465547ce3803
myIsprime(97) # ==> true --> :); 97 is prime
# ╔═╡ 6bd55608-02f5-4b95-8ad9-597abdf106b6
md"
###### Sample from the [First 100,008 Primes](https://primes.utm.edu/lists/small/100000.txt)
"
# ╔═╡ 4d06eb30-b117-4b02-b04c-6c459a73bcd8
myIsprime(99989) # ==> true --> :)
# ╔═╡ 33fb902c-bece-4876-a731-3eaf7ffe2ff4
myIsprime(99991) # ==> true --> :)
# ╔═╡ 9bc0eb42-79b0-4d7b-b472-07abae671439
myIsprime(99993) # ==> false --> :); because 3 * 33331 = 99993
# ╔═╡ 57187f78-2aeb-4284-90da-b529a298eb91
myIsprime(100003) # ==> true --> :)
# ╔═╡ 8d69e081-99d6-404a-805a-5d12821c5f5a
md"
###### [Carmichael numbers](https://en.wikipedia.org/wiki/Carmichael_number) (= [Fermat *pseudo*primes](https://en.wikipedia.org/wiki/Fermat_pseudoprime))
A *Carmichael number* will *usually* pass *Fermat primality test* even though it is *not* actually prime. But this test $myIsprime$ is searching divisors *exhausively*, so its perfect but not effective. *...there are $20.138.200$ Carmichael numbers between $1$ and $10^{21}$*. [(*Wikipedia*)](https://en.wikipedia.org/wiki/Carmichael_number)
The first *seven* Carmichael numbers *were all found by the Czech mathematician Václav Šimerka in 1885* ([*Wikipedia*](https://en.wikipedia.org/wiki/Carmichael_number)):
$561, 1105, 1729, 2465, 2821, 6601, 8911$
"
# ╔═╡ 6a7a2391-9cbc-44ef-83f8-5252c1454ac2
md"
###### 1st Carmichael number
"
# ╔═╡ fa620300-3307-4bda-97de-1c49fbf58d61
myIsprime(561) # ==> false --> :)
# ╔═╡ b5278a66-dbca-44d0-96ad-bd8e24e90ee3
md"
###### 2nd Carmichael number
"
# ╔═╡ 81147945-ae09-4fa3-a9ef-288a8c42744e
myIsprime(1105) # ==> false --> :)
# ╔═╡ e555ade7-e8f8-4158-8b68-0af132d8861a
md"
###### 3rd Carmichael number
"
# ╔═╡ a37e9857-fc51-4fdc-b8a5-50f6607f52bf
myIsprime(1729) # ==> false --> :)
# ╔═╡ d641ab56-f97a-4cab-820d-2d1ce7b3c546
md"
###### 4th Carmichael number
"
# ╔═╡ 04a9798b-fc72-455e-be71-62842167b292
myIsprime(2465) # ==> false --> :)
# ╔═╡ 1c6fbf70-c098-44cc-ac84-28acc220da7d
md"
###### 5th Carmichael number
"
# ╔═╡ af7c8d34-5bf3-4e5a-851e-4b7d13b5f6b4
myIsprime(2821) # ==> false --> :)
# ╔═╡ 8cec6bc7-37fd-43b0-81de-863d6a93e153
md"
###### 6th Carmichael number
"
# ╔═╡ 28eca35a-ff7a-4f07-84dd-08d8329f8fd2
myIsprime(6601) # ==> false --> :)
# ╔═╡ fbc3c07c-7db4-4c69-add6-5135d33c2ebd
md"
###### 7th Carmichael number
"
# ╔═╡ eb73d8b4-c26f-49a7-9790-49c72fbf85e9
myIsprime(8911) # ==> false --> :)
# ╔═╡ 113f62c9-e1b7-44f9-a113-d90be226ef51
md"
###### 8th Carmichael number
"
# ╔═╡ 25dad7c6-453d-4849-aeb1-cc56ac1c6faa
myIsprime(10585) # ==> false --> :)
# ╔═╡ d6bc8877-7093-4c3c-a256-ab1409183866
md"
###### 9th Carmichael number
"
# ╔═╡ 3e9d4b35-0e9d-4a23-874c-82ed4edee8fb
myIsprime(15841) # ==> false --> :)
# ╔═╡ be8ba70d-fd85-40d9-bd91-79907b148439
md"
###### 10th Carmichael number
"
# ╔═╡ 26c58152-1e0a-45e4-bbf9-effefff6c695
myIsprime(29341) # ==> false --> :)
# ╔═╡ 7d4832ca-cd8c-49af-aa3e-df0104dc677c
md"
###### 11th Carmichael number
"
# ╔═╡ ac8e5704-a619-4545-95c3-313f4e8d6585
myIsprime(41041) # ==> false --> :)
# ╔═╡ 714d49ef-ede2-439a-acac-d8065ca22771
md"
###### 12th Carmichael number
"
# ╔═╡ 6811e773-beb5-442a-88b6-61de219eb71d
myIsprime(46657) # ==> false --> :)
# ╔═╡ 534db6f2-c7ea-4e67-9a29-66a1063a3722
md"
###### 13th Carmichael number
"
# ╔═╡ 6e8b2888-2c41-4868-a451-0f68a0256056
myIsprime(52633) # ==> false --> :)
# ╔═╡ 376f22c8-6323-4f4f-ba68-d4066b0c08f5
md"
###### 14th Carmichael number
"
# ╔═╡ b9994c20-d8b9-43e0-839f-0727c2280868
myIsprime(62745) # ==> false --> :)
# ╔═╡ 38ac1686-0f3a-4969-907d-16e9ab98ea65
md"
###### 15th Carmichael number
"
# ╔═╡ dcc6e279-3775-404e-adec-23fe32388032
myIsprime(63973) # ==> false --> :)
# ╔═╡ 42f795ad-acd6-49a1-8328-61217325c991
md"
###### 16th Carmichael number
"
# ╔═╡ 148f7d98-dbd5-473c-ab4a-6fa0559f8413
myIsprime(75361) # ==> false --> :)
# ╔═╡ e698ce81-6cd0-4a9a-931a-5994bf91dc39
md"
---
###### 3.2 Variation $isprime2$ of $isprime$: *Generic* (*Polymorphic*) function
with *local* function definitions
"
# ╔═╡ 9979c1e4-349b-4e0a-87a0-39de45ecdf4c
function myIsprime2(n)
#-------------------------------------------
remainder = % # idiomatic Julia '%'
#-------------------------------------------
square(x) = ^(x, 2) # infix operator '^'
#-------------------------------------------
divides(a, b) = ==(remainder(b, a), 0)
#-------------------------------------------
smallest_divisor() = find_divisor(2)
#-----------------------------------------------------------------------------
function find_divisor(test_divisor)
>(square(test_divisor), n) ? n :
divides(test_divisor, n) ? test_divisor :
find_divisor( +(test_divisor, 1))
end # function find_divisor
#-----------------------------------------------------------------------------
n == smallest_divisor()
end # function myIsprime2
# ╔═╡ e6ac837a-adc0-4114-9b09-edb43ab4d5bb
myIsprime2(1) # ==> true --> :); some say 1 is not a prime
# ╔═╡ c5ed48aa-74b8-4a68-958f-10aca604ea5b
myIsprime2(2) # ==> true --> :)
# ╔═╡ dca62763-1e1d-48c8-97dc-bc47bca6f2d3
myIsprime2(3) # ==> true --> :)
# ╔═╡ 692e04a0-ea2c-4ce4-bee2-2d3d4736259b
myIsprime2(4) # ==> false --> :)
# ╔═╡ df838016-92fc-4255-be0a-85fb0990bce7
myIsprime2(5) # ==> true --> :)
# ╔═╡ cb61b117-245b-4845-808c-70deb40f7061
myIsprime2(6) # ==> false --> :)
# ╔═╡ f2741526-1906-4ea4-8800-17d27a74ed18
myIsprime2(97) # ==> true --> :)
# ╔═╡ 218cc9d7-3c37-44a6-82b8-fecdf8e6fc02
myIsprime2(99989) # ==> true --> :)
# ╔═╡ d803272a-0090-495d-93a2-d56d2d9b6858
myIsprime2(99991) # ==> true --> :)
# ╔═╡ 23cdee96-73a6-4bdc-ba7d-627f84821ea5
myIsprime2(99993) # ==> false --> :); because 3 * 33331 = 99993
# ╔═╡ 01ba9d22-919c-4887-919e-88dcfbc7a03c
myIsprime2(100003) # ==> true --> :)
# ╔═╡ 4cb5f5b4-c0a5-4bac-93e8-1b5069e3863a
md"
###### [Carmichael numbers](https://en.wikipedia.org/wiki/Carmichael_number) (= [Fermat *pseudo*primes](https://en.wikipedia.org/wiki/Fermat_pseudoprime))
A *Carmichael number* will *usually* pass *Fermat primality test* even though it is *not* actually prime. But this test $myIsprime2$ is searching divisors *exhausively*, so its perfect but not effective.
"
# ╔═╡ c9f0881e-95d4-44ab-8a1b-84b9b1636478
md"
###### 1st Carmichael number
"
# ╔═╡ 894d5371-66dd-4cfa-83ad-a1a5ef96fe81
myIsprime2(561) # ==> false --> :)
# ╔═╡ 9ccc6095-fb94-4bbe-8b79-adf38cacadcf
md"
###### 2nd Carmichael number
"
# ╔═╡ 000041cd-e527-46ba-bdbb-c899ccc0567d
myIsprime2(1105) # ==> false --> :)
# ╔═╡ d90c320e-c578-45ea-bc98-f54b63a83a4b
md"
###### 3rd Carmichael number
"
# ╔═╡ b7f68f0f-6dc4-4131-947e-ec88ad3827b3
myIsprime2(1729) # ==> false --> :)
# ╔═╡ 0ed8c711-5530-454f-8c7f-8f72fe90032b
md"
###### 4th Carmichael number
"
# ╔═╡ bfaca850-d65f-4354-9149-1d97f23cad8b
myIsprime2(2465) # ==> false --> :)
# ╔═╡ b9fee51c-774b-474d-a508-962df966b5b0
md"
###### 5th Carmichael number
"
# ╔═╡ 7906ee7f-c102-42c6-beff-aa08d6f2ebdf
myIsprime2(2821) # ==> false --> :)
# ╔═╡ 977e806a-b361-4d54-bf12-ea7e46e64bc6
md"
###### 6th Carmichael number
"
# ╔═╡ 5eed380c-d7f3-4a64-ab58-b2598e4925e1
myIsprime2(6601) # ==> false --> :)
# ╔═╡ f0189ee0-ee8f-450f-a909-73050adfb965
md"
###### 7th Carmichael number
"
# ╔═╡ c7159488-3e73-4327-bd66-0f80cdc5937b
myIsprime2(8911) # ==> false --> :)
# ╔═╡ c6a5e5cb-c266-4315-88a6-e02990d7e21c
md"
###### 8th Carmichael number
"
# ╔═╡ 29d544ec-ca31-403e-bb47-ac2cb75bb211
myIsprime2(10585) # ==> false --> :)
# ╔═╡ 2febaefa-c436-4b93-b312-30c2dfda4f9a
md"
###### 9th Carmichael number
"
# ╔═╡ a2800255-5583-448f-9b89-8ec4b1bdfe47
myIsprime2(15841) # ==> false --> :)
# ╔═╡ 1624efd8-abb1-47b4-b42d-b190c0a93099
md"
###### 10th Carmichael number
"
# ╔═╡ 7297f795-c99d-46f0-bf6b-30ba3954b4d4
myIsprime2(29341) # ==> false --> :)
# ╔═╡ 8edd5779-2050-4255-aee0-1b28edff2540
md"
###### 11th Carmichael number
"
# ╔═╡ b1cc4859-166c-4836-817f-6bfe480978ca
myIsprime2(41041) # ==> false --> :)
# ╔═╡ ec478f2e-1b78-44f6-a99f-7822fb1594b8
md"
###### 12th Carmichael number
"
# ╔═╡ 186e941a-d81b-4f64-a974-346268d74654
myIsprime2(46657) # ==> false --> :)
# ╔═╡ e5fa1992-0f35-467f-a113-66a0f6a45b48
md"
###### 13th Carmichael number
"
# ╔═╡ f7288e68-eb85-4a4a-b744-d7fb82446d17
myIsprime2(52633) # ==> false --> :)
# ╔═╡ b9732f72-9344-45f4-9b44-09ff50c1c373
md"
###### 14th Carmichael number
"
# ╔═╡ 0e33cbc9-159f-4d26-b945-490e3922037a
myIsprime2(62745) # ==> false --> :)
# ╔═╡ f150a27b-728c-47ef-945d-62331de9f474
md"
###### 15th Carmichael number
"
# ╔═╡ ecb30694-9742-4519-8b51-c1d520a7335e
myIsprime2(63973) # ==> false --> :)
# ╔═╡ 6cece608-0153-4415-963f-e94bc54c32c2
md"
###### 16th Carmichael number
"
# ╔═╡ 134fb4b5-29d2-41be-8ccd-5dff7a7d6cf0
myIsprime2(75361) # ==> false --> :)
# ╔═╡ 374eccff-3888-4760-9ce7-09b388beaed0
md"
---
###### 3.3 [Fermat's Little Theorem](https://en.wikipedia.org/wiki/Fermat%27s_little_theorem) (SICP version)
*If $n$ is a prime number and $a$ is any positive integer less than $n$, then $a$ raised to the $n$th power is congruent to $a$ modulo $n$* (SICP, 1996; [2016, p.67](https://web.mit.edu/6.001/6.037/sicp.pdf); Cormen et. al., 2022, p.932, 944):
$\boxed{\text{If } isprime(n) \text{ then } \forall a \in \{a|1 < a < n\}: a^n \equiv a\ (\bmod n)}$
where $a$ is called *basis* and where:
$a^n \equiv a\ (\bmod n) \text{ is an abbreviation for: } a^n\ (\bmod n) = a\ (\bmod n).$
Below is another (shorter) version of the theorem, which is *almost perfect* (Cormen et al., 2022, p.944).
"
# ╔═╡ 84499038-4ed6-46fc-a6d1-70a5964c3f3e
md"
---
###### 3.3.1 *Modular-Exponentiation* $expmod1$
*To implement the Fermat test, we need a procedure that computes
the exponential of a number modulo another number* (SICP, 1996; 2016, p.67; Cormen et al., 2022, p.934):
$a^b\ (\bmod n) = \cases{1 \;\;\;\;\;\;\;\;\;\;\,\text{ if }\ b = 0 \\(a^{b/2})^2 \;\;\text{ if } (b > 0) \land iseven(b) \\ a \cdot a^{b-1} \;\text{ if }(b > 0) \land isodd(b)}$
"
# ╔═╡ 99f32ce2-e7c5-4a0a-b466-d7ecf5ef6af4
md"
... with *self-defined* type FloatOrSigned
"
# ╔═╡ 8bbdd3d4-9a2b-478d-8ba4-7ca3a10aa86e
FloatOrInteger = Union{AbstractFloat, Signed}
# ╔═╡ 0af40eda-a6fd-4fb3-9d77-129c02d66b79
# a^b mod n or baseA^expB mod n
#idiomatic Julia-code '%', '^2'
function expmod1(baseA::Signed, expB::FloatOrInteger, modN::Signed)
#---------------------------------------------------------
square(x) = x^2
#---------------------------------------------------------
remainder = %
#---------------------------------------------------------
even(n)::Bool = remainder(n, 2) == 0
#---------------------------------------------------------
if expB == 0
1
elseif even(expB)
remainder(square(expmod1(baseA, expB/2, modN)), modN)
else
remainder(baseA * expmod1(baseA, expB-1, modN), modN)
end # if
end # function expmod1
# ╔═╡ bff175b8-7637-40ae-84a5-3047a5c71d30
md"
$2^3\ (\bmod 3)= 8\ (\bmod 3) = 2$
"
# ╔═╡ 5ee7549a-dff7-4809-b63c-ebf572ad10eb
expmod1(2, 3, 3)
# ╔═╡ 65c473e2-c57a-4045-bd41-1f0ee046ef0f
md"
$2^5\ (\bmod 5)= 32\ (\bmod 5) = 2$
"
# ╔═╡ 770100ce-9cd8-4892-a3c0-d15c97192b5b
expmod1(2, 5, 5)
# ╔═╡ d83d7003-094c-4e99-8762-d642156581d0
md"
$3^5\ (\bmod 5)= 243\ (\bmod 5) = 3$
"
# ╔═╡ 247589bd-70b9-4677-bf8f-472aa4d57804
expmod1(3, 5, 5)
# ╔═╡ 24479ece-4777-4c18-a846-682c2364486d
md"
---
###### 3.3.2 *Modular-Exponentiation* $expmod2$
"
# ╔═╡ 708d6883-e125-489e-bbe7-76ff0fa59aa6
#idiomatic Julia-code '%', '^2', 'convert'
function expmod2(baseA::Signed, expB::FloatOrInteger, modN::Signed)::Signed
even(x) = x%2 == 0
if expB == 0
1
elseif even(expB)
%(expmod2(baseA, expB/2, modN)^2, modN)
else
%(baseA * expmod2(baseA, expB-1, modN), modN)
end # if
end # function expmod2
# ╔═╡ 80174e42-0389-4efe-b80a-97625860c4dd
expmod2(2, 3, 3) # ==> 2 --> :)
# ╔═╡ 18bcfbd7-be4e-45d3-8e80-0953a422368d
expmod1(2, 5, 5) # ==> 2 --> :)
# ╔═╡ d3002fe5-3fc9-493e-93ca-e0e243845d68
expmod2(3, 5, 5) # ==> 3 --> :)
# ╔═╡ cebc2513-0f94-41df-ab52-9bfe6815bb8d
md"
---
###### 3.3.3 *Fermat Test* (SICP, 1996; 2016, p.68) $fermat\_test1$
*The Fermat test is performed by choosing at random a number a between 1 and n - 1 inclusive and checking whether the remainder modulo n of the nth power of a is equal to a. The random number a is chosen using the procedure random, which we assume is included as a primitive in Scheme. Random returns a nonnegative integer less than its integer input. Hence, to obtain a random number between 1 and n - 1, we call random with an input of n - 1 and add 1 to the result:* (SICP, 1996; 2016, p.68)
"
# ╔═╡ e57b3eb9-a394-4253-8f23-76ff51bc1998
histogram([rand(1:10) for i in 1:10E2], bins=1:11)
# ╔═╡ 8d905cbe-ca91-413c-ae68-0c7613685b5b
rand(1:3)
# ╔═╡ 7079c8a5-af5b-4083-b494-aaaf0ed1336a
# idiomatic Julia-code with 'rand'
function fermat_test1(n::Signed)::Bool
#--------------------------------------------------------------------
random(n) = rand(1:n) - 1 # MIT-Scheme: 0 ≤ random(n) ≤ n-1
#--------------------------------------------------------------------
# try_it(a) = expmod2(a, n, n) == a # short form as in SICP
try_it(a) = expmod2(a, n, n) == mod(a, n) # long form as in theorem
#--------------------------------------------------------------------
try_it(1 + random(n-1))
end # fermat_test1
# ╔═╡ cd1ed62c-086a-4d92-b625-dfce3362f1c6
fermat_test1(2) # ==> true --> :)
# ╔═╡ f3dc6cb1-d360-4684-a860-09da9180456a
fermat_test1(3) # ==> true --> :)
# ╔═╡ f22888d3-26a2-4dc5-af18-4f9e5e05ba91
fermat_test1(4) # ==> false --> :) ; switches between true / false
# ╔═╡ e7b85f55-93e2-41d5-8a0c-2948e943c3f2
fermat_test1(5) # ==> true --> :)
# ╔═╡ ac859543-dae4-49de-9afc-ea9c5d6600e8
fermat_test1(6) # ==> false --> :) ; switches between true / false
# ╔═╡ 49ad573f-078e-4ebb-a1e2-da27e6c46a1d
fermat_test1(7) # ==> true --> :)
# ╔═╡ 643e4865-a682-4ef5-9c04-1e058d60f01b
fermat_test1(9) # ==> false --> :)
# ╔═╡ 6d344ac0-c073-45ae-8d74-532ae7001a92
fermat_test1(97) # ==> true --> :)
# ╔═╡ c9403479-b7e2-4391-a058-541501434de0
md"
---
###### 3.3.4 *Fermat Test* with *Replications*: $fast\_prime1$
*The following procedure runs the test a given number of times, as specified by a parameter. Its value is true if the test succeeds every time, and false otherwise.* (SICP, 1996; 2016, p.68)
"
# ╔═╡ 8a3e42bf-6c51-4201-b894-a96bc3b8c6b6
function fast_prime1(n::Signed, times::Signed)::Bool
if times == 0
true
elseif fermat_test1(n)
fast_prime1(n, times-1)
else
false
end # if
end # fast_prime1
# ╔═╡ cbdc8f90-774b-49ad-8736-033e4d98fbd3
fast_prime1(2, 10) # ==> true --> :)
# ╔═╡ ce83ac3a-974f-4c2a-b407-16f89055c66c
fast_prime1(3, 10) # ==> true --> :)
# ╔═╡ b90b682f-e083-44b9-93e8-2d7f143c8386
fast_prime1(4, 10) # ==> false --> :)
# ╔═╡ ff66c235-6d17-42ad-bbfd-7b4cd5dd0bac
fast_prime1(4, 100) # ==> false --> :)
# ╔═╡ a8d176f8-af2e-4fbe-a4ae-2588443f80e1
fast_prime1(5, 10) # ==> true --> :)
# ╔═╡ 2408748e-f577-48b4-8828-e8bc59914a7a
fast_prime1(6, 10) # ==> false --> :)
# ╔═╡ 13d75f8a-edc4-4042-8473-7eb24764b404
fast_prime1(7, 10) # ==> true --> :)
# ╔═╡ 5474b2e3-ca51-42ce-9a61-f8a22420a162
fast_prime1(9, 19) # ==> false --> :)
# ╔═╡ e2116f5a-636b-4194-9e5f-acffa2656a1c
fast_prime1(11, 10) # ==> true --> :)
# ╔═╡ b2432745-1d5e-4f47-8c8e-dfa360bf3923
fast_prime1(97, 10) # ==> true --> :)
# ╔═╡ 4ca1868f-c694-47fe-be38-6f1899c8e902
fast_prime1(99989, 10) # ==> true --> :)
# ╔═╡ 41e11d01-7a92-4663-8b79-3d7302cbc985
fast_prime1(99991, 10) # ==> true --> :)
# ╔═╡ 2275dc7a-43d9-47aa-a5ab-e1b53a8c7df2
fast_prime1(99993, 10) # ==> false --> :); 99993 = 3*33331
# ╔═╡ 442e1767-4055-4a49-9f46-665240a0537d
fast_prime1(100003, 10) # ==> true --> :)
# ╔═╡ cac5241d-8e7c-4c78-87ff-9df4d12632af
md"
###### Carmichael numbers fool Fermat's Test
"
# ╔═╡ 8cbdd3f2-e7d5-4e78-9d0b-34b43ac14b40
md"
###### 1st Carmichael number
"
# ╔═╡ eb712016-f15a-43aa-96b0-e129bff835f8
fast_prime1(561, 100) # ==> true --> :( ; test was fooled
# ╔═╡ f7513770-7b6c-4bc8-b9fb-ca306f0cb9d8
md"
###### 2nd Carmichael number
"
# ╔═╡ f05ef05e-f1be-4706-b950-77e273c7884e
fast_prime1(1105, 100) # ==> true --> :( ; test was fooled
# ╔═╡ c433c8bd-43e4-463a-8ff2-7b6bb70aaef3
md"
###### 3rd Carmichael number
"
# ╔═╡ 681da4df-ecb6-40ec-a5b8-03d49b25822c
fast_prime1(1729, 100) # ==> true --> :( ; test was fooled
# ╔═╡ 5bcdaf47-9be0-4824-bd99-da274e30a360
md"
###### 4th Carmichael number
"
# ╔═╡ cb5d584d-5928-4223-9daf-139397b2e380
fast_prime1(2465, 100) # ==> true --> :( ; test was fooled
# ╔═╡ 72c045cb-9655-4f2c-8340-4d9b19e11fce
md"
###### 5th Carmichael number
"
# ╔═╡ 6882300e-3e56-4eac-b7de-b6dd78a4d5cb
fast_prime1(2821, 100) # ==> true --> :( ; test was fooled
# ╔═╡ 3351208c-94a4-4cc4-a3d0-851b0986010a
md"
###### 6th Carmichael number
"
# ╔═╡ 55293415-c1d3-4f8d-ab45-098e4a86d295
fast_prime1(6601, 100) # ==> true --> :( ; test was fooled
# ╔═╡ 8408cbef-007b-4b64-a7d3-be4944113217
md"
###### 7th Carmichael number
"
# ╔═╡ 2a484f43-8e7b-4e6e-88c0-1d27a45cf4aa
fast_prime1(8911, 100) # ==> true --> :( ; test was fooled
# ╔═╡ 076c461a-7a4b-42d7-a7fd-1006a8fa0f89
md"
###### 8th Carmichael number
"
# ╔═╡ 182fe756-65b6-43ee-8238-a8e6bd7950ee
fast_prime1(10585, 100) # ==> true --> :( ; test was fooled
# ╔═╡ caef03c1-fe96-4504-a100-3478312e79c0
md"
###### 9th Carmichael number
"
# ╔═╡ bd896dca-d317-431a-89f3-a57dfef64f8f
fast_prime1(15841, 100) # ==> true --> :( ; test was fooled
# ╔═╡ 742b6b98-d66b-41e5-96bf-8f3882047e6a
md"
###### 10th Carmichael number
"
# ╔═╡ ca245186-4ad0-4fd1-8cbb-7310f455b3cb
fast_prime1(29341, 100) # ==> true --> :( ; test was fooled
# ╔═╡ 9d643e66-9504-4a40-81d3-690c2df98c5f
md"
###### 11th Carmichael number
"
# ╔═╡ 7aab20ba-4685-47fe-8337-1037c686909f
fast_prime1(41041, 100) # ==> true --> :( ; test was fooled
# ╔═╡ 7477cc56-adfd-44bb-959b-819dc63fba9c
md"
###### 12th Carmichael number
"
# ╔═╡ 8b82b7b8-ba1f-4f7f-897e-c13f78cdf847
fast_prime1(46657, 100) # ==> true --> :( ; test was fooled
# ╔═╡ 7c3b5bfb-fb6e-4d63-80f6-7a8a7dd1c0a6
md"
###### 13th Carmichael number
"
# ╔═╡ 8fcf9b3f-e7d3-4e33-b671-da484cfa07cc
fast_prime1(52633, 100) # ==> true --> :( ; test was fooled
# ╔═╡ 96f3e1b1-442e-47ef-9306-b1debafb1e5c
md"
###### 14th Carmichael number
"
# ╔═╡ 1e6174e0-ed2e-4167-8a5f-0361b6b2ff1b
fast_prime1(62745, 100) # ==> true --> :( ; test was fooled
# ╔═╡ 060e3768-239c-4e18-a8d9-bfb585e6c17e
md"
###### 15th Carmichael number
"
# ╔═╡ b0fe5cd7-e660-4dae-bd09-c826cfcd60f2
fast_prime1(63973, 100) # ==> true --> :( ; test was fooled
# ╔═╡ 31b1939d-94bc-4c06-ace7-dbe70e2fb5cb
md"
###### 16th Carmichael number
"
# ╔═╡ a3febf5c-1b67-499a-96c3-77d94d7eaae2
fast_prime1(75361, 100) # ==> true --> :( ; test was fooled
# ╔═╡ f3e9ad78-79f2-4aeb-8113-c8f50d05e68e
md"
---
###### 3.6 [Fermat's Little Theorem](https://en.wikipedia.org/wiki/Fermat%27s_little_theorem) (short version, e.g. Cormen et al., 2022, p.932)
If *basis* $a$ and *test candidate* $n$ are *coprime* then $gcd(a, n)==1$. In this case we can divide both sides of the equation
$a^n\ (\bmod n) = a\ (\bmod n)$
by $a\ (\bmod n)$. Now, we get:
$a^{n-1}\ (\bmod n) = a^{1-1}\ (\bmod n) = a^0\ (\bmod n) = 1\ (\bmod n) = 1.$
So:
$a^{n-1}\ (\bmod n) = 1.$
We can include the condition $gcd(n, a) = 1$ as a precondition in *Fermat*'s test. If we *know* that $isprime(n)$ is *true* this check is redundant. In other cases we can use it as a *quick-check*. If $gcd(n, a) > 1$ then we know that $isprime(n)$ is *false* and $n$ is *composite*. Otherwise we are *uncertain* whether $n$ is *prime* or *pseudoprime* (=*Carmichael*) and we have to go on with testing.
Now we can write the *short* version of *Fermat's Little Theorem* as:
$\boxed{\text{If } isprime(n) \text{ then } \forall a \in \{a|(gcd(n, a) = 1) \land (1 < a < n) \}: a^{n-1}\ (\bmod n) = 1}$
When we abbreviate the conclusion $\text{ then } ... 1$ as:
$FTest(+|n, \forall a) =_{def} \forall a \in \{a|(gcd(n, a) = 1) \land (1 < a < n) \}: a^{n-1} (\bmod n) = 1.$
We can write the *short* version of *Fermat*'s Little Theorem as:
$\boxed{\text{If } isprime(n) \text{ then } FTest(+|n, \forall a)}.$
"
# ╔═╡ 88f02453-317c-4162-bb18-764715015a0b
# idiomatic Julia-code with 'rand'
function fermat_test2(n::Signed)::Bool
#--------------------------------------------------------------------
random(n) = rand(1:n) - 1 # MIT-Scheme: 0 ≤ random(n) ≤ n-1
#--------------------------------------------------------------------
# try_it(a) = expmod2(a, n, n) == a # short form as in SICP
try_it(a) = (gcd(n, a) == 1) && (expmod2(a, n-1, n) == 1)
#--------------------------------------------------------------------
try_it(1 + random(n-1))
end # fermat_test2
# ╔═╡ 5b9a1a53-2081-4778-aab6-a60fcfbef3b6
function fast_prime2(n::Signed, times::Signed)::Bool
if times == 0
true
elseif fermat_test2(n)
fast_prime2(n, times-1)
else
false
end # if
end # fast_prime2
# ╔═╡ d77a7757-c72d-4e8f-93c9-3009700ab975
fast_prime2(2, 100) # ==> true --> :)
# ╔═╡ 595b9dda-541b-4732-a294-affce2b0c4e5
fast_prime2(3, 100) # ==> true --> :)
# ╔═╡ c8d3613a-6d2b-4901-9321-aad8a99179cb
fast_prime2(4, 100) # ==> false --> :)
# ╔═╡ fda84f34-1d43-446b-8c27-145b51812455
fast_prime2(5, 100) # ==> true --> :)
# ╔═╡ 9a45cf8d-4d2b-48cc-9fc9-e0c4fd9468da
fast_prime2(6, 100) # ==> false --> :)
# ╔═╡ 02659ecf-235a-4391-9998-167a69f6714e
fast_prime2(7, 100) # ==> true --> :)
# ╔═╡ 7b7b0882-61be-4b76-96e8-18cb0d872ef6
fast_prime2(9, 100) # ==> false --> :)
# ╔═╡ 2a6189a7-885d-42c1-bddb-c53891cfe972
fast_prime2(11, 100) # ==> true --> :)
# ╔═╡ 0d3af18b-2cb0-4d25-b0b0-9df9f64caf9c
fast_prime2(97, 100) # ==> true --> :)
# ╔═╡ 8e12127d-e814-4001-9100-77f679ed4c65
fast_prime2(99989, 100) # ==> true --> :)
# ╔═╡ 293c9a6d-74cc-4e48-9386-4a3b946d5f20
fast_prime2(99991, 100) # ==> true --> :)
# ╔═╡ 9a08b4be-03d3-4dee-aa7c-233a0427da15
fast_prime2(99993, 100) # ==> false --> :), because 3*33331 = 99993
# ╔═╡ fa52d319-05b9-4798-9b9b-91d6ff907569
fast_prime2(100003, 100) # ==> true --> :)
# ╔═╡ f64f11be-05fa-44a5-a14f-28044bac53aa
md"
###### Carmichael numbers *usually* fool Fermat's Test, but not $fast\_prime2$
"
# ╔═╡ 864506fb-9979-4cf9-a17d-fdfb0d88f5bf
md"
###### 1st Carmichael number
"
# ╔═╡ d021796a-f02e-42ab-911f-b018e13797b8
fast_prime2(561, 100) # ==> false --> :) ; test could *not* be fooled
# ╔═╡ baf18a7f-7661-4fba-8cfa-5add5ea6d5e2