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 A053381 Maximal number of linearly independent smooth nowhere-zero vector fields on a (2n+1)-sphere. 6

%I

%S 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,

%T 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,

%U 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

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

%C 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.

%C 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

%C a(n-1) is multiplicative. - _Christian G. Bower_, Jun 03 2005

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

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

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

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

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

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

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

%H T. D. Noe, <a href="/A053381/b053381.txt">Table of n, a(n) for n = 0..10000</a>

%F 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.

%F 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

%F a(n) = A209675(n+1) - 1. - _Reinhard Zumkeller_, Mar 11 2012

%p 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:

%p 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

%t 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 *)

%o (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); }

%Y For another version see A003484. Cf. A047680, A001676.

%Y Cf. A047530, A220466.

%K nonn,nice,easy,mult

%O 0,2

%A _Warren D. Smith_, Jan 06 2000

%E More terms from _James A. Sellers_, Jun 01 2000

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Last modified August 17 11:40 EDT 2017. Contains 290635 sequences.