## Kervaire invariant: Why dimension 126 especially difficult?

Is there any resource that might help non-experts gains some understanding of why the Kervaire invariant problem remains open now only in dimension $126$? ($126 =2^7-2=2^{j+1}-2$; whether $\theta_j=\theta_6$ exists in the "$128$-stem"), i.e., why the celebrated Hill-Hopkins-Ravenel proof technique fails in this last remaining case? I read a delightful exposition by Erica Klarreich ("Mathematicians solve 45-year-old Kevaire Invariant Puzzle," The Best Writing on Mathematics, 2010, 373ff, Simons Foundation link), which piqued my interest. But Michael Hopkins' online presentation slides announcing the result in 2009 ("Applications of algebra to a problem in topology") are beyond my ken. Perhaps this an area too abstruse for all but the experts? I'd appreciate pointers to expositions. Thanks!

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The problem is complicated and either you dive into it or you won't understand anything at all, there's no way of posting a short enough answer to your question. I'd recommendo you to read Snaith's book on the topic an then the papers by Hill-Hopkins-Ravanel and Akhmetev. – Fernando Muro Jul 1 at 0:57
There was also a Bourbaki Seminar talk by Haynes Miller, the report is available on arXiv arxiv.org/abs/1104.4523 – quid Jul 1 at 1:01
Thanks, quid, that is a great resource, exactly what I was seeking. And thanks, Fernando, for the reference to (I guess?): Snaith, Victor P. (2009), "Stable homotopy around the Arf-Kervaire invariant," Progress in Mathematics, 273, Birkhäuser Verlag. – Joseph O'Rourke Jul 1 at 1:31

I'll give a shot at an answer. The relevant dimensions are of the form $2^j-2$. For $j\leq 4$, it is easy and classical that we can construct manifolds of Kervaire invariant one. The problem was reduced'' from differential topology to pure stable homotopy theory by Browder in 1969. Direct calculational methods in homotopy theory were used by Barratt, Jones, and Mahowald to construct a cell complex that can be used to solve the homotopy theory problem and prove that such manifolds also exist in dimensions 30 and 62. I believe a construction of such a manifold has been worked out in dimension 30, but that has certainly not been done in dimension 62. Periodicity phenomena play a huge role in modern stable homotopy theory, and a crucial feature of the Hill, Hopkins, Ravenel proof is a periodicity of order $2^8 = 256$. That enables them to solve the stable homotopy problem and prove there is no manifold of Kervaire invariant one for $j\geq 8$. The reasons $j=7$ is so hard are several. Nobody has a really good reason for guessing which way the answer will go. There is no reason to expect a relevant periodicity of order $2^7$. Direct calculation of the Adams spectral sequence through dimension $126$ is just plain hard: the calculations blow up. There is a chance that the methodology of Barratt, Jones, and Mahowald might extend to prove existence (if that is how the answer turns out!), but it will probably be much harder to prove nonexistence (if that is the answer).
I disagree with the statement that there is no reason for guessing which way the answer will go. There is a good reason, which I learned from Atiyah, to guess that the answer in dimension 126 is the same as in the lower dimensions 62, 30, 14,... Namely, there exist remarkable manifolds in dimensions 128, 64, 32, and 16. These are the octooctonionic projective plane (see math.ucr.edu/home/baez/octonions/node19.html), the quateroctonionic projective plane (see math.ucr.edu/home/baez/octonions/node18.html), the bioctonionic projective plane, and finally $OP^2$ itself. – AndrĂ© Henriques Jul 1 at 19:26