fast construction of symbolic sums

[rwst]

In legendre_P::eval_pari there is this:

return sum(b * arg**a for a, b in enumerate(pol))

Due to GiNaC::add::eval mechanics it has quadratic performance, fortunately eased by smart pointer operations. Much better however would be to have the iterator add to an GiNaC::exvector and construct the GiNaC::add from that. There is GExVector.push_back and g_add_construct in the Pynac interface already available.

Component: symbolics

Issue created by migration from https://trac.sagemath.org/ticket/23417

[rwst]

Description changed:

--- 
+++ 
@@ -3,4 +3,4 @@
 ```
 return sum(b * arg**a for a, b in enumerate(pol))
 ```
-Due to `GiNaC::add::eval` mechanics it has quadratic performance, fortunately eased by smart pointer operations. Much better however would be to have the iterator add to an `GiNaC::epvector` and construct the `GiNaC::add` from that. The obvious internal Pynac code just needs an interface in `libs/pynac` and this then used by `legendre_P::eval_pari`.
+Due to `GiNaC::add::eval` mechanics it has quadratic performance, fortunately eased by smart pointer operations. Much better however would be to have the iterator add to an `GiNaC::exvector` and construct the `GiNaC::add` from that. There is `GExVector.push_back` and `g_add_construct` in the Pynac interface already available.

[rwst]

comment:2

Duplicate of #24553.