# CoCoA:Subst

(Redirected from ApCoCoA-1:Subst)

## Subst

substitute values for indeterminates

### Description

The first form of this function substitutes F_i for X_i in the

expression E. The second form is a shorthand for the first in the

case of a single indeterminate. When substituting for the indeterminates in order, it is easier to use <ttref>Eval</ttref>.

#### Example

```  Use R ::= Q[x,y,z,t];
F := x+y+z+t^2;
Subst(F,x,-2);
t^2 + y + z - 2
-------------------------------
Subst(F,x,z/y);
(yt^2 + y^2 + yz + z)/y
-------------------------------
Subst(F,[[x,x^2],[y,y^3],[z,t^5]]);
t^5 + y^3 + x^2 + t^2
-------------------------------
Eval(F,[x^2,y^3,t^5]); -- the same thing as above
t^5 + y^3 + x^2 + t^2
-------------------------------
MySubst := [[y,1],[t,3z-x]];
Subst(xyzt,MySubst);  -- substitute into the function xyzt
-x^2z + 3xz^2
-------------------------------
```

### Syntax

```Subst(E:OBJECT,X,F):OBJECT
Subst(E:OBJECT,[[X_1,F_1],...,[X_r,F_r]]):OBJECT

where each X or X_i is an indeterminate and each F or F_i is a number,
polynomial, or rational function.
```

```   <type>polynomial</type>
<type>ratfun</type>
```