basefield real: subst for parameters does not work

[gorzel]

Compare:

> ring r0 = (0,t),x,dp;
> poly f = tx+2;
> subst(f,x,1/2);
(1/2t+2)
> subst(f,t,1/2);   // OK in char 0 for parameters
1/2*x+2

> ring rr = (real,t),x,dp;
> poly f = tx+2;
> subst(f,x,1/2);
(2+t*0.5)
> subst(f,t,1/2);    // fails for parameters with real
   ? ringvar/par expected
   ? error occurred in or before STDIN line 10: `subst(f,t,1/2);`

[hannes]

This is as it should be:

ring rr = (real,t),x,dp;
rr;
//   characteristic : 0 (complex:6 digits, additional 6 digits)
//   1 parameter    : t 
//   minpoly        : (t^2+1)
//   number of vars : 1
//        block   1 : ordering dp
//                  : names    d
//        block   2 : ordering C
}}
and one should not be able to substitute sqrt(-1) by 1/2.

[gorzel]

War mir bisher nicht bekannt, dass real mit Parameter sich automatisch wie complex verhaelt.

> ring rr = (real,t),x,dp;
> rr;
//   characteristic : 0 (complex:6 digits, additional 6 digits)
//   1 parameter    : t 
//   minpoly        : (t^2+1)
//   number of vars : 1
//        block   1 : ordering dp
//                  : names    x
//        block   2 : ordering C
> ring rc = complex,x,dp;
> rc;
//   characteristic : 0 (complex:6 digits, additional 6 digits)
//   1 parameter    : i 
//   minpoly        : (i^2+1)
//   number of vars : 1
//        block   1 : ordering dp
//                  : names    x
//        block   2 : ordering C
> ring rct = (complex,t),x,dp;
> rct;
//   characteristic : 0 (complex:6 digits, additional 6 digits)
//   1 parameter    : t 
//   minpoly        : (t^2+1)
//   number of vars : 1
//        block   1 : ordering dp
//                  : names    x
//        block   2 : ordering C
}


This feature is not explained in the manual.
http://www.singular.uni-kl.de/Manual/latest/sing_29.htm#SEC39