Adem relations (${\mathrm{Sq}}^a{\mathrm{Sq}}^b={\sum }_t\left(\genfrac{}{}{0ex}{}{b-t-1}{a-2t}\right){\mathrm{Sq}}^{a+b-t}{\mathrm{Sq}}^t(a<2b)$)

These mod-two identities rewrite nonadmissible products of Steenrod squares. For example, ${\mathrm{Sq}}^1{\mathrm{Sq}}^1=0$, ${\mathrm{Sq}}^1{\mathrm{Sq}}^2={\mathrm{Sq}}^3$, and ${\mathrm{Sq}}^1{\mathrm{Sq}}^3=0$. In general the sum ranges over $0\le t\le \lfloor a/2\rfloor$ and coefficients are reduced modulo two.