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Substitution

Parts of an expression may be replaced by other expressions using subst(a,b,c): a is substituted for b in c. This is a powerful function with many uses.

First, it may be used to insert numbers for variables, in the example for in der formula .

>> syms x
>> a=2*sqrt(x)*exp(-x^2);
>> subst(3,x,a)
ans = 4.275E-4


Second, one can replace a symbolic variable by a complex term. The expression is automatically updated to the canonical format. In the following example is inserted for in .

>> syms x,z
>> p=x^3+2*x^2+x+7;
>> subst(z^3+2,x,p)
ans = z^9+8*z^6+21*z^3+25


Finally, the term b itself may be a complex expression (in the example ). Jasymca then tries to identify this expression in c (example: ). This is accomplished by solving the equation for the symbolic variable in b (example: ), and inserting the solution in c. This does not always succeed, or there may be several solutions, which are returned as a vector.

>> syms x,y,z
>> c=x^3*z/sqrt(z^2+1);
>> d=subst(y,z^2+1,c)
d = [ x^3*sqrt(y-1)/sqrt(sqrt(y-1)^2+1)
-x^3*sqrt(y-1)/sqrt(sqrt(y-1)^2+1) ]
>> d=trigrat(d)
d = [ x^3*sqrt(y-1)/sqrt(y)
-x^3*sqrt(y-1)/sqrt(y) ]


Next: Simplifying and Collecting Expressions Up: Symbolic Transformations Previous: Symbolic Transformations
Helmut Dersch
2009-03-15