Coding infinite many series ?

clc
close all

alpha=1/2;
beta=1/3;
T=1;
t=0:T;
a=2; %x1^0
b=1; %x2^0
a11=1;
a12=2;
a21=3;
a22=4;

for (k=0:'k<K':k++)
   l=2*k+2;
    A=zeros(:,l);
for (j=1: j<J; j++)
    x=0;y=0;p=0;q=0;
    for (i=0:  i<k: i++)
    x=x+alpha*A[2*i+1]+ beta*A[2*i];
    y=y+alpha*A[2*i+1]+ beta*A[2*i+2]};
    p=p+(a11^sum_odd[A]*a22^sum_even[A]*t^(x+k*(alpha+beta)))/gamma(x+k*(alpha+beta)+1);
    
    q=q+(a11^sum_odd[A]*a22^sum_even[A]*t^(y+(k+1)*(alpha+beta)+k*beta)))/gamma(y+(k+1)*(alpha+beta)+k*beta+1);
    
    for n=1; n<=l; n++
        A[n]=j
        function f=mydouble(sum_odd[A]+sum_even[A])
              f.value=A;
          end
        function update(A)
        end
   x1(t)=a*a12^k.*a21^k.*p+b*a12^(k+1).*a21^k*q;
    end
    end
end 
end

[Walter Roberson]

Your for loops have the wrong syntax.

https://www.mathworks.com/help/matlab/ref/for.html

Note: MATLAB does not have any ++ operator either.