gap-forum
David Ponce wrote:
gap> R := PolynomialRing(Rationals,["x"]); PolynomialRing(..., [ x ]) gap> gap> f:=x^2+1; 1+x^2 gap> f in R; falsereturn "true"? Am I using the "in" command wrong?
Where did you define the *variable* x? Your first line assigns a
*name* x to Indeterminate(Rationals,1). (If the above is your entire
GAP run. Otherwise it assigns the name "x" to
Indeterminate(Rationals, whichever number), see below.) The line
"f:=x^2+1;" should have produced an error, unless your excerpt is part
of a bigger GAP run, where x was assigned before.
Also, with `PolynomialRing', you have to be very careful with repeated
calls. See the GAP run below:
gap> R := PolynomialRing(Rationals,["x"]);
PolynomialRing(..., [ x ])
gap> f:=x^2+1;
Variable: 'x' must have a value
gap> var:=Indeterminate(Rationals,1);
x
gap> f:=var^2+1;
1+x^2
gap> f in R;
true
gap> R := PolynomialRing(Rationals,["x"]);
PolynomialRing(..., [ x ])
gap> f in R;
false
gap> var:=Indeterminate(Rationals,2);
x
gap> f:=var^2+1;
1+x^2
gap> f in R;
true
gap>
Sincerely,
Luc Teirlinck.
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