Adem Relations

Relations in the definition of a Steenrod Algebra which state that, for $i<2j$,

$$Sq^i\circ Sq^j(x)=\sum_{k=0}^{\left\lfloor{i}\right\rfloor } {{j-k-1}\choose{i-2k}}Sq^{i+j-k}\circ Sq^{k}(x),$$

where $f\circ g$ denotes function Composition and $\left\lfloor{i}\right\rfloor$ is the Floor Function.

See also Steenrod Algebra