# How to integral a composite function in yalmip?

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Tong Liu on 13 Jul 2019
Commented: Tong Liu on 15 Jul 2019
sdpvar x1 x2 x3 u1
% syms x1 x2 x3 u1
d=x1+x2;
y=x1+x2+u1+d;
z=y+3;
t=int(z,y,0,y)
hello everyone...I have define a integration by yalmip, but there are some error..
If replace by syms, there are still errors.

infinity on 13 Jul 2019
Hello,
You define z as a function of x1, x2, and u1. But, you apply the integral function to compute the integral of z over the interval [0, y] and told the integral function that the variable of z is also y. It is not correct.
Let image that what you are doing like trying to compute Tong Liu on 15 Jul 2019
Thank you for your answer. As you know, z is also a function of y. Why can't I intergral according to y directly? Is that only the final variable like x and u can be used for integration?
Thanks!
infinity on 15 Jul 2019
Hello,
In case, we have function z(y) = y + 3 then we can compute the integral of z with respect to y as simple by
syms y a
z = y + 3;
res = int(z,y,0,a)
where "a" is upper limit of the integral.
But, in your given code, you defined z as function of x1, x2, u1 and parameter d. Matlab can not understand that z is a function y.
Tong Liu on 15 Jul 2019
Get it. Thank you for your kindly help!