A053381 Maximal number of linearly independent smooth nowhere-zero vector fields on a (2n+1)-sphere.

1, 3, 1, 7, 1, 3, 1, 8, 1, 3, 1, 7, 1, 3, 1, 9, 1, 3, 1, 7, 1, 3, 1, 8, 1, 3, 1, 7, 1, 3, 1, 11, 1, 3, 1, 7, 1, 3, 1, 8, 1, 3, 1, 7, 1, 3, 1, 9, 1, 3, 1, 7, 1, 3, 1, 8, 1, 3, 1, 7, 1, 3, 1, 15, 1, 3, 1, 7, 1, 3, 1, 8, 1, 3, 1, 7, 1, 3, 1, 9, 1, 3, 1, 7, 1, 3, 1, 8, 1, 3, 1, 7, 1, 3, 1, 11, 1, 3, 1, 7, 1, 3

Offset

0,2

Comments

The corresponding terms for a 2n-sphere are all 0 ("you can't comb the hair on a billiard ball"). The "3" and "7" come from the quaternions and octonions.

b(n) = a(n-1): b(2^e) = ((e+1) idiv 4) + 2^((e+1) mod 4) - 1, b(p^e) = 1, p>2. - Christian G. Bower, May 18 2005

a(n-1) is multiplicative. - Christian G. Bower, Jun 03 2005

Links

T. D. Noe, Table of n, a(n) for n = 0..10000

J. Frank Adams, Vector fields on spheres, Topology, 1 (1962), 63-65.

J. Frank Adams, Vector fields on spheres, Bull. Amer. Math. Soc. 68 (1962) 39-41.

J. Frank Adams, Vector fields on spheres, Annals of Math. 75 (1962) 603-632.

A. Hurwitz, Uber die Komposition der quadratischen formen, Math. Annalen 88 (1923) 1-25.

M. Kervaire, Non-parallelizability of the sphere for n > 7, Proc. Nat. Acad. Sci. USA 44 (1958) 280-283.

J. Milnor, Some consequences of a theorem of Bott, Annals Math. 68 (1958) 444-449.

J. Radon, Lineare Scharen Orthogonaler Matrizen, Abh. Math. Sem. Univ. Hamburg 1 (1922) 1-14.

Formula

Let f(n) be the number of linearly independent smooth nowhere-zero vector fields on an n-sphere. Then f(n) = 2^c + 8d - 1 where n+1 = (2a+1) 2^b and b = c+4d and 0 <= c <= 3. f(n) = 0 if n is even.

a((2*n+1)*2^p-1) = A047530(p+1), p >= 0 and n >= 0. a(2*n) = 1, n >= 0, and a(2^p-1) = A047530(p+1), p >= 0. - Johannes W. Meijer, Jun 07 2011

a(n) = A209675(n+1) - 1. - Reinhard Zumkeller, Mar 11 2012

Asymptotic mean: Limit_{m->oo} (1/m) * Sum_{k=1..m} a(k) = 10/3. - Amiram Eldar, Nov 29 2022

Maple

with(numtheory): for n from 1 to 601 by 2 do c := irem(ifactors(n+1)[2, 1, 2], 4): d := iquo(ifactors(n+1)[2, 1, 2], 4): printf(`%d, `, 2^c+8*d-1) od:
nmax:=101: A047530 := proc(n): ceil(n/4) + 2*ceil((n-1)/4) + 4*ceil((n-2)/4) + ceil((n-3)/4) end: for p from 0 to ceil(simplify(log[2](nmax))) do for n from 0 to ceil(nmax/(p+2))+1 do A053381((2*n+1)*2^p-1) := A047530(p+1): od: od: seq(A053381(n), n=0..nmax); # Johannes W. Meijer, Jun 07 2011, revised Jan 29 2013

Mathematica

a[n_] := Module[{b, c, d, rho, n0}, n0 = 2*n; b = 0; While[BitAnd[n0, 1] == 0, n0 /= 2; b++]; c = BitAnd[b, 3]; d = (b - c)/4; rho = 2^c + 8*d; Return[rho - 1]]; Table[a[n], {n, 1, 102}] (* Jean-François Alcover, May 16 2013, translated from C *)

Prog

(C) int MaxLinInd(int n){ /* Returns max # linearly indep smooth nowhere zero * vector fields on S^{n-1}, n=1, 2, ... */ int b, c, d, rho; b = 0; while((n & 1)==0){ n /= 2; b++; } c = b & 3; d = (b - c)/4; rho = (1 << c) + 8*d; return( rho - 1); }
(C, gcc/clang) int MaxLinInd(int n) { int b = __builtin_ctz(n); return (1<<b%4) + b/4*8 - 1; } /* Jeremy Tan, Apr 09 2021 */

Crossrefs

For another version see A003484. Cf. A189995, A001676.

Cf. A047530, A220466.

Sequence in context: A227984 A021991 A112132 * A038712 A354587 A065745

Adjacent sequences: A053378 A053379 A053380 * A053382 A053383 A053384

Keyword

nonn,nice,easy,mult

Author

Warren D. Smith, Jan 06 2000

Extensions

More terms from James A. Sellers, Jun 01 2000

Status

approved