Let $X$ be a topological space. A class $a\in H_n(X;\mathbb Z)$ is said to be primitive if $a\not = m b$ for every integer $m>1$ and $b \in H_n(X;\mathbb Z)$. Let $\Delta_*:H_n(X;\mathbb Z)\to H_n(X\times X;\mathbb Z)$ be the induced map in homology of the diagonal map $\Delta(x)=(x,x)$. I want to show that if $a\in H_n(X;\mathbb Z)$ is primitive then $\Delta_*(a)=a\otimes 1+1\otimes a$
I know that in a field $F$ we can use a simplified Kunneth formula in the following way: $H_n(X\times X;F)=\sum_{i+j=n}{H_i(X;F)\otimes H_j(X;F)}=H_n(X;F)\otimes H_0(X;F)\oplus H_0(X;F)\otimes H_n(X;F)\oplus \sum_{i+j=n,i\not =0,i\not =n}{H_i(X;F)\otimes H_j(X;F)}$ But i can't go further!