Torsion in Bbso

Pacific Journal of Mathematics

TORSION IN BBSO

JAMES D.STASHEFF

Vol. 28, No. 3 May 1969 PACIFIC JOURNAL OF MATHEMATICS Vol. 28, No. 3, 1969

TORSION IN BBSO

JAMES D. STASHEFF

The cohomology of BBSO, the classifying space for the stable Grassmanian BSO, is shown to have torsion of order precisely 2r for each natural number r. Moreover, the elements of order 2r appear in a pattern of striking simplicity.

Many of the stable Lie groups and homogeneous spaces have torsion at most of order 2 [1, 3, 5]. There is one such space, however, with interesting torsion of higher order. This is BBSO = SU/Spin which is of interest in connection with Bott periodicity and in connection with the J-homomorphism [4, 7]. By the notation Sϊ7/Spin we mean that BBSO can be regarded as the fibre of B Spin -+BSU or that, up to homotopy, there is a fi.bration

SZ7-> BBSO-+ £Spin induced from the universal SU bundle by B Spin —>BSU. The mod

2 cohomology H*(BBSO; Z2) has been computed by Clough [4]. The purpose of this paper is to compute enough of H*(BBSO; Z) to obtain the mod 2 Bockstein spectral sequence [2] of BBSO.

Given a ring R, we shall denote by R[x{ \iel] the polynomial ring on generators x{ indexed by elements of a set I. The set I will often be described by an equation or inequality in which case i is to be understood to be a natural number. Similarly E(xt \iel) will denote the exterior algebra on generators x{. In this case, we will need only R — Z2. Let us recall the results on B Spin as given by Thomas [6] and on BBSO as given by Clough [4].

j H*(B Spin; Z2) e* Z2[w{ \ ί Φ 2 + 1]

where w{ is (the image of) the Stiefel-Whitney class wt.

H*(B Spin; Z) ~ Z[Qt \ i > 0] φ f where 2T = 0 and Qi e H4i.

H*(BBSO; Z2) ** E(e{ \ ί ^ 3)

1 j where e^H and is the image of wt if i Φ 2 + 1 while e23 +1 maps to an indecomposable element in H*(SU; Z2).

Now let βEr denote the mod 2 Bockstein spectral sequence of 1 1 BBSO [2]. In particular, βE2 = Ker Sq /Jm Sq . Now Sq'w2i = w2i+1 in

BSO and Sq'w2i+ι = 0 while Sq%j = 0 in B Spin. We will see that

677 678 JAMES D. STASHEFF

1 e2j+1 can be chosen to have Sq e25+ι = 0 except for Sq% = e4. Thus

βE2 = E(e3eiy β22+<, vu+1 \ i > 0)

where v4i+1 = e2ie2i+1 except v2j+1 = e2J+1; j > 1.

THEOREM 1.

βEr & E(e5e4 e2r, e2r+i, vu+1 \ i > 0)

To prove Theorem 1, we will exhibit torsion of order 2r for all r.

THEOREM 2. In H*(BBS0; Z), we have

r r+1 2 Q2r Φ 0 and 2 Q2r - 0 .

H*(BBS0; Z2). We recall some of Clough's observations on 2i+ί H*(BBS0; Z2). We know H*(SU; Z2) = E{y, \ ί > 1) where yt e H transgresses universally to the mod 2 reduction of the Chern class c< and hence to the image of w\ in B Spin. Thus w\ — 0 in BBSO for j i ^ 2 + 1 and y2j is the restriction of a class β2i+1+1. In particular 23 2 2 23 2 since Sq (w2J_1+1) = (^2y+1) we can take e2J+1 to be Sq^^Sq " 2 S

H*(BBS0, Z). Consider BBSO as the fibre of B Spin->BSU. The latter map factors: B Spin-^-> BSO—>BSU. Recall that

H*(BSU; Z) = Z[ct \ i > 1] and H*(BS0; Z) = ZIP,} 0 T

7 where T is the torsion ideal, 2T = 0, c2ί+1 maps into Γ and c2i maps to P;. To determine Im (#*(£ Spin)) in H*(BBS0), we need to know 7Γ*[P,] in i

THEOREM 3 (Thomas [6]). // i is not a power of 2, π*P{ =

i - 2\ r > 0, π*P2i = 2Q2J + QJ. - π*Φ2j. π*P, = 2Qlβ

LEMMA. π*02i maps iwίo Im Tc H*(BBS0). Proof. H*(BS0; Z) maps onto Im Γ in H*{BBSO) since H*(BSU) maps onto the ^[P^] part. Since 7Γ*P,. goes to zero in BBSO, we have in H*(BBS0; Z) TORSION IN BBSO 679

r 2Q2j = - Q) + t where 2t = 0 and j = 2 . 20, = 0 . By iteration we find

2 r = ± 2Q2rQ2r_1 ... Q2(QX) = 0 .

To determine the order of Q2i in BBSO, consider Γ(u | 2u = 0), a divided polynomial algebra on a single generator u of dimension 4 and order 2; i.e., additively JΓ has generators y^u) in dimension 4i and the mul- tiplication table is yi(u)y3 (u) = (ί, j)yi+j(u) where (i, i) is the binomial coefficient {(i + j)l/iljl}. In particular i!7»(^) = u\ We construct a map / from Im {H*{B Spin; Z) -^H*(BBS0; Z)) 1 j to i" by mapping T to zero, Q{ to zero for i Φ 2 and Q2i to

-Ύ,(f(Q2j-i)) with /(Qx) - w. Since 2Q2J = - Q^ί, + π*Φ2j, and

ord/(Qa,) = 2ord/(Q2i-i) = 2>ord/(Q1) = 2^ .

j+1 j+1 Thus the order of Q2j is at least 2 and that 2 Q2i is in fact zero we have already seen. Thus we have 2r torsion for each r. From the exact cohomology sequence derived from 0—>Z >Z—>Z2r—>0, we see that Q2r-i =

β™rxr for some class xr e H*(BBS0; Z2r), where /9~ is the connecting +ι homomorphism H*( Z2r)-> H* { Z).

LEMMA. (/5^.^r)2 = dr(xr)2 where ( )2 means reduction mod 2.

oo r 1 Proo/. Recall how dr is defined: dr(x) = (/92 (^)/2 - )2. From the commutativity of the diagram

2r-l

Zd > ZJ > ZJ2

r 1 oo it follows that β~ = 2 - /S2 r. In particular, dr(xr)2 = (Q2r-ή2. Ac- cording to Thomas, (Q2r-i)2 = π*(w2r+i + ^2r+i) where ψ2T+i is decomposable.

In particular, (Qί)2 = TΓ4. We prove Theorem 2 by induction. Since

ι Sq w2i = w2i+1 and S^Wgi+i = 0 ,

1 1 y we know Sg ^ = β2ί+1 and Sq e2i+1 = 0 unless i = 2 . Since we have 23 1 2 1 2 chosen e2J+1 — Sq " Sq e3, we have Sq e2j+ι = (e2j-i+1) = 0 for 680 JAMES D. STASHEFF

j ^> 2. For j = l, we have Sq^ = e4 because β4 = (Qx)2 which is in 1 the image of Sq since 2Qι = 0. Thus

βE2 = Ker

= E(e,e4) (g) #(e2ίβ2ί+11 2 < i ^= 20 ® E(eiS+ι, e2J+1 \j^2).

Since cZ2(x2)2 = (Q2)2 = e8, we must have £2 = β3β4.

In general dr(xr)2 = (Q2r~ι)2 = e2r+1 modulo decomposables. Now consider H*(BBSO; Q). Since H*(BSO; Q) = Q[Pi] with the usual dia- gonal m*^) = Σi+*=< ^ 0 P*, we have H*(BBSO; Q) = E(Rd where 4i+1 dim iϋ; € £Γ . Thus βE* = E(Sii+1) and the only possibility is

j S4i+1 = e2ie2i+1 i Φ 2 ,

modulo terms decomposable in terms of the S4ί+1. This leaves e3β4

e2r as the only possibility for xr, i.e., dr(β3e4 ••• e2r) = e2r+i mod decomposables as claimed.

BIBLIOGRAPHY

1. A. Borel, La cohomologie mod 2 des certains espaces homogenes, Comm. Math. Helv. 27 (1953), 165-197. 2. W. Browder, Torsion in H-spaces, Ann. of Math. 74 (1961), 24-51. 3. H. Cartan, Seminaire—1959/60, Exposes 3, 17, Paris, 1961. 4. R. R. Clough, Calculation of H*(Bim{j); Z*) (to appear). 5. E. Dyer and R. K. Lashof, A topological proof of the Bott periodicity theorem, Ann. Mat. Pura Appl. (4) 54 (1961), 231-254. 6. E. Thomas, On the cohomology groups of the classifying space for the stable spinor group, Bol. Soc. Mat. Mex. (1962), 57-69. 7. G. W. Whitehead, On the homotopy groups of spheres and rotation groups, Ann. of Math. 43 (1942), 634-640.

Received May 3, 1968. Research supported in part by NSF Grant GP-6101. The author is an Alfred P. Sloan Fellow. PACIFIC JOURNAL OF MATHEMATICS

EDITORS H. ROYDEN J. DUGUNDJI Stanford University Department of Mathematics Stanford, California University of Southern California Los Angeles, California 90007

R. R PHELPS RICHARD ARENS University of Washington University of California Seattle, Washington 98105 Los Angeles, California 90024

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