OFFSET
0,2
COMMENTS
Conjecture. a(n) = 2n*3^(n-1)+1.
If conjecture is true then limsup(A137576(n)/n)=infinity while liminf(A137576(n)/n)=2 with a realization on primes.
a(n) is also the number of edges in the graph generated from the n-dimensional hypercube (plus 1) in the following manner: connect all (d + 1)-dimensional faces to the d faces that are incident. Each d-dimensional face should be incident on (n - d) (d + 1)-dimensional faces. [Roy Liu (royliu(AT)cs.ucsd.edu), Jul 26 2010]
LINKS
Wikipedia, Hypercube
FORMULA
Sum_{m = 0}^{n} 2^(n - m) * binomial(n,m) is the number of m-dimensional faces in the n-dimensional hypercube. Consequently, Sum_{m = 0..n} (n - m) * 2^(n - m) * binomial(n,m) gives the number of incidence edges, which yields said sequence minus 1. The recurrence relation is: a(n) = 3 * a(n - 1) + 2 * 3^(n - 1) - 2. [Roy Liu (royliu(AT)cs.ucsd.edu), Jul 26 2010]
Empirical G.f.: (1-4*x+7*x^2)/(1-7*x+15*x^2-9*x^3). [Colin Barker, Jan 09 2012]
PROG
(PARI) a137576(n) = my(t); sumdiv(2*n+1, d, eulerphi(d)/(t=znorder(Mod(2, d))))*t-t+1;
a(n) = a137576((3^n-1)/2); \\ Michel Marcus, Dec 18 2018
CROSSREFS
KEYWORD
nonn
AUTHOR
Vladimir Shevelev, May 26 2008
EXTENSIONS
More terms from Michel Marcus, Dec 18 2018
STATUS
approved