Tighten power-trace bound of odd loop Hafnian (#362)

* Tighten power-trace bound of odd loop Hafnian In the inner-loop, `powtrace[i // 2]` reaches a maximum at `n == i` according to the loop range, hence the power-trace need not compute any additional power traces. * Update CHANGELOG.md --------- Co-authored-by: Nicolas Quesada <[email protected]>
XanaduAI · Jul 18, 2023 · 8e43ac1 · 8e43ac1
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diff --git a/.github/CHANGELOG.md b/.github/CHANGELOG.md
@@ -8,6 +8,9 @@
 
 ### Improvements
 
+
+* Tighten power-trace bound of odd loop Hafnian. [(#362)](https://github.com/XanaduAI/thewalrus/pull/362)
+
 * Simplifies the internal working of Bloch-Messiah decomposition [(#363)](https://github.com/XanaduAI/thewalrus/pull/338). 
 
 * Simplifies the internal working of Williamson decomposition [(#366)](https://github.com/XanaduAI/thewalrus/pull/338). 
@@ -20,7 +23,7 @@
 
 This release contains contributions from (in alphabetical order): 
 
-Nicolas Quesada
+Gregory Morse, Nicolas Quesada
 
 ---
 

diff --git a/thewalrus/_hafnian.py b/thewalrus/_hafnian.py
@@ -264,7 +264,7 @@ def f_loop_odd(AX, AX_S, XD_S, D_S, n, oddloop, oddVX_S):  # pragma: no cover
     count = 0
     comb = np.zeros((2, n + 1), dtype=np.complex128)
     comb[0, 0] = 1
-    powtrace = charpoly.powertrace(AX, n + 1)
+    powtrace = charpoly.powertrace(AX, n // 2 + 1)
     for i in range(1, n + 1):
         if i == 1:
             factor = oddloop