Take the 2-minute tour ×
MathOverflow is a question and answer site for professional mathematicians. It's 100% free, no registration required.

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?

share|improve this question
add comment

2 Answers

up vote 18 down vote accepted

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.

share|improve this answer
    
Thankyou, this is a very clear explanation! –  Jon Beardsley Feb 8 '12 at 19:37
9  
Yes, very nice. An alternative reason is that for the mod $2$ Moore spectrum any map $R\wedge R\to R$ must induce the zero map $\pi_0(R)\otimes \pi_0(R)\to\pi_0(R)=Z/2$ because $\pi_0(R\wedge R)=Z/4$. –  Tom Goodwillie Feb 9 '12 at 2:19
3  
Tom, I think your proof is better. –  Charles Rezk Feb 9 '12 at 6:35
add comment

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.

share|improve this answer
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.