crash with ultraspherical polynomials

[rwst]

%time _=ultraspherical(10000,7/5,x)
...
RuntimeError: ECL says: Memory limit reached. Please jump to an outer pointer, quit program and enlarge the
memory limits before executing the program again.

Currently Maxima is called with ultraspherical (Gegenbauer) polynomials but already a naive Python implementation is faster:

sage: def u(n,a):
    return sum((-1)^k*prod(i for i in srange(a, n-k+a))/factorial(n-2*k)/factorial(k)*(2*x)^(n-2*k) for k in range(floor(n/2)+1))
....:
sage: %time _=u(10000,7/5)
CPU times: user 1min 43s, sys: 128 ms, total: 1min 43s
Wall time: 1min 43s

and the Maxima version would be not even expanded. A direct Pynac implementation needs 25 seconds.

Dependencies: pynac-0.6.5

Component: symbolics

Issue created by migration from https://trac.sagemath.org/ticket/20428

[rwst]

Description changed:

--- 
+++ 
@@ -16,4 +16,4 @@
 CPU times: user 1min 43s, sys: 128 ms, total: 1min 43s
 Wall time: 1min 43s
 ```
-and the Maxima version would be not even expanded. A direct Pynac implementation should need only seconds.
+and the Maxima version would be not even expanded. A direct Pynac implementation needs 25 seconds.

[rwst]

Dependencies: pynac-0.6.5

[rwst]

comment:2

Fixed in #20312.