Merge pull request #45 from mredenti/calculus-deriv-dft

Fix bug in computation of derivative with DFT

boututils/calculus.py

@@ -44,18 +44,20 @@ def deriv(*args, **kwargs):
 
     n = var.size
     if periodic:
+        # assume uniform grid 
+        dx = x[1] - x[0]
         # Use FFTs to take derivatives
         f = rfft(var)
         f[0] = 0.0  # Zero constant term
         if n % 2 == 0:
             # Even n
             for i in arange(1, old_div(n, 2)):
-                f[i] *= 2.0j * pi * float(i) / float(n)
+                f[i] *= 2.0j * pi * float(i) / (float(n)*dx)
             f[-1] = 0.0  # Nothing from Nyquist frequency
         else:
             # Odd n
             for i in arange(1, old_div((n - 1), 2) + 1):
-                f[i] *= 2.0j * pi * float(i) / float(n)
+                f[i] *= 2.0j * pi * float(i) / (float(n)*dx)
         return irfft(f)
     else:
         # Non-periodic function