Notational question on Kunneth Formula for de Rham cohomology I got to learn the Kunneth Formula for de Rham cohomology as following.
$$H^n(X\times Y)=\sum_{n=p+q} H^p(X)\otimes H^q(Y).  $$
And I could find same notation from https://www.encyclopediaofmath.org/index.php/K%C3%BCnneth_formula.
Actually, it is quite unfamiliar to use $\sum$ for spaces. I think it should be $\oplus$ instead of $\sum$. It might be stupid question and maybe I am wrong and missing something. I am looking for some clarification for it!  
And clear explanation for this would be appreciated! Thanks in advance.

Yes, you're right. Indeed we are talking about the direct sum over these spaces. Some people use $\sum$ rather than $\bigoplus$ in this situation. My advisor actually does the same thing. I suspect that it's a more old-fashioned means of writing  the expression. But, anyway, the expression should be read as:
$$ H^n(X\times Y)=\bigoplus_{p+q=n} H^p(X)\otimes H^q(Y)$$
as you propose.
