Difficulties with the mod 2 Moore Spectrum - MathOverflow

Difficulties with the mod 2 Moore Spectrum

2012-02-08T18:38:51Z

I have been informed that there is a reference out there which specifically details what goes wrong with the mod 2 Moore spectrum, i.e. why it is not $A_\infty$ or something? I do not know the details, but I am interested in this from the point of view of trying to understand how to come up with the correct notion of ideals of spectra (in the sense of Smith and others), especially the ideal generated by multiplication by 2 on the sphere spectrum. I think it is a paper by Neeman. Does anyone know of this paper, or of other papers which might detail this situation carefully?

Thanks!

PS Is this question appropriate, as it is only a reference request, in the strongest sense of the phrase?

Answer by Charles Rezk for Difficulties with the mod 2 Moore Spectrum

2012-02-08T19:18:18Z

The actual statement is much stronger than you suggest, namely: the mod 2 Moore spectrum does not admit a unital multiplication (even if it is non-associative). I don't know a reference so I'll sketch the proof:

Let $R$ be a spectrum with unital product, with unit map $\eta\colon S^0\to R$ and product map $\mu: R\wedge R\to R$. Then it is straightforward to show that if $n\eta=0$ in $\pi_0R$ for some integer $n$, then $n\cdot\mathrm{id}_R: R\to R$ is homotopic to the null map as well. (The key point is that proving this uses the existence of $\mu:R\wedge R\to R$ such that $\mu\circ (\eta\wedge \mathrm{id}_R) = \mathrm{id}_R$, but nothing about associativity of such $\mu$.)

If $R$ is the mod $2$ Moore spectrum, with $\eta: S^0\to R$ the generator of $\pi_0R$, then you calculate that:

$\pi_0R = Z/2$, but
$\pi_2R = Z/4$,

from which it follows that

$2\eta=0$,
$2\mathrm{id}_R\neq 0$.

Therefore no such unital multiplication $\mu$ on $R$ can exist.

Answer by Jeff Smith for Difficulties with the mod 2 Moore Spectrum

2012-02-19T03:45:12Z

An argument from the old days. A unital multiplication gives a non-trivial splitting of $R\wedge R$ whereas its mod 2 cohomology is indecomposable as a module over the steenrod algenbra.