Finding cocycles that square to zero - MathOverflow most recent 30 from http://mathoverflow.net 2013-05-23T17:31:23Z http://mathoverflow.net/feeds/question/8703 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/8703/finding-cocycles-that-square-to-zero Finding cocycles that square to zero Tim Perutz 2009-12-12T18:44:46Z 2009-12-12T22:16:21Z <p>Suppose $x$ is a chosen class in the singular cohomology (integer coefficients) of a space $X$. I'm thinking primarily of classes of odd degree on a simply connected space. What are necessary conditions (besides $x^2=0$) for the existence of a cocycle representing $x$ whose cup-square equals zero as a cocycle? Sufficient conditions?</p> <p>Take your pick of the precise form of the question: you can fix a cochain model for cup products before or after choosing $x$, or even allow a DGA quasi-isomorphic to the singular cochains on $X$.</p> <p>You may feel inclined to mutter "Steenrod square" or "Massey product" - but which, and why?</p> http://mathoverflow.net/questions/8703/finding-cocycles-that-square-to-zero/8704#8704 Answer by Mariano Suárez-Alvarez for Finding cocycles that square to zero Mariano Suárez-Alvarez 2009-12-12T19:02:39Z 2009-12-12T22:16:21Z <p>The triple product $\langle x,x,x\rangle$ has to contain zero.</p> <p>Indeed, if $a$, $b$, $c$ are odd cohomology classes such that $ab=0$ and $bc=0$, to compute the triple product $\langle a, b, c\rangle$, one picks representative cocycles $\alpha$, $\beta$ and $\gamma$, then picks cochains $\delta$ and $\eta$ such that $\alpha\beta=d\delta$ and $\beta\gamma=d\eta$, and then observes that $\tau=\alpha\eta+\delta\gamma$ is a cocycle. Then $\tau$ is a representative of $\langle a,b,c\rangle$ in an appropriate quotient of the cohomology group which contains the class of $\tau$.</p> <p>In your case, suppose we can represent the class $x$ by a cocycle $\xi$ such that $\xi^2=0$. Then if we take $a=b=c=x$, we can take $\alpha=\beta=\gamma=\xi$ and $\delta=\eta=0$, so that $\tau=0$, that is, $0\in \langle x,x,x\rangle$.</p> <p>In fact, all Massey products $\langle x,x,\dots,x\rangle$ ("Massey powers"?) have to be zero, by a similar computation---see the book by McCleary on spectral sequences, chapter 8, for a speedy description of these.</p>