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2.2.15 Substitutions
To substitute a list of numbers or polynomials for the indeterminates (in the order specified by the definition of the ring), one may use the function Eval . To substitute out of order, use the function Subst .

Example 
  Use R ::= QQ[x,y,z];
  F := x^2+y^2+z^2;
  Eval(F,[1]);  -- substitute x=1
y^2 + z^2 + 1
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  Eval(F, [1,2,3]);  -- substitute x=1, y=2, z=3
14
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  Type(It); -- [1,2,3] has NumIndets() rational components
RAT
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  Subst(F, y, 2);  -- substitute y=2
x^2 + z^2 + 4
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  Eval(F, [x,2,z]); -- same as above
x^2 + z^2 + 4
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  Subst(F, [[y,y^2],[z,z^2]]);  -- substitute y^2 for y, z^2 for z
y^4 + z^4 + x^2
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  Eval(Ideal(F), [x^2,z]); -- substitute x^2 for x, z for y
Ideal(x^4 + 2z^2)
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