adding jacobi.py

pykat/math/jacobi.py

+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function
+from __future__ import unicode_literals
+
+import numpy as np
+import scipy.special
+
+def jacobi(n,a,b,x):
+  """ Implementation of Jacobi fucntion using binominal coefficients.
+  This can handle values of alpha, beta < -1 which the special.eval_jacobi
+  function cannot."""
+  P=0.0
+  for s in np.arange(0,n+1):
+    P=P+scipy.special.binom(n+a,s) * scipy.special.binom(n+b,n-s) * (x-1.0)**(n-s) * (x+1.0)**s
+  P=P*0.5**n
+  return P