= Cellular chains of a product of finite CW complexes
{title2=$d(e^p\times f^q)=de^p\times f^q+(-1)^pe^p\times df^q$}
The product cells of finite <CW complexes> give the <tensor product> of their <cellular chain complexes>. The boundary formula is the displayed signed product rule. The sign comes from moving the outward normal of the second factor past the $p$ oriented tangent directions of the first factor. This is the cellular counterpart of the <Eilenberg–Zilber theorem>.
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