OFFSET
1,1
COMMENTS
Term a(6) has 135 decimal digits and a(7) has 404 decimal digits. - Andrew Howroyd, Jun 19 2017
LINKS
Andrew Howroyd, Table of n, a(n) for n = 1..7
Hanlin Chen, Renfang Wu, Guihua Huang, and Hanyuan Deng, Independent Sets on the Towers of Hanoi Graphs, Ars Mathematica Contemporanea, volume 12, number 2, 2017, pages 247-260. Section 3, i_n = a(n).
Eric Weisstein's World of Mathematics, Hanoi Graph
Eric Weisstein's World of Mathematics, Independent Vertex Set
Eric Weisstein's World of Mathematics, Vertex Cover
MATHEMATICA
{1, 3, 3, 1} . # & /@ NestList[Function[{h, i, j, k}, {h^3 + 6 h^2 i + 9 h i^2 + 3 h^2 j + 2 i^3 + 6 h i j, h^2 i + 4 h i^2 + 2 h^2 j + h^2 k + 8 h i j + 3 i^3 + 4 i^2 j + 2 h j^2 + 2 h i k, h i^2 + 4 h i j + 2 i^3 + 7 i^2 j + 2 h i k + 3 h j^2 + 4 i j^2 + 2 i^2 k + 2 h j k, i^3 + 6 i^2 j + 9 i j^2 + 3 i^2 k + 2 j^3 + 6 i j k}] @@ # &, {1, 1, 0, 0}, 4]
PROG
(PARI)
\\ Here h0..h3 is independent sets with 0..3 of the 3 apex vertices occupied.
Next(h0, h1, h2, h3) = {[h0^3 + 6*h0^2*h1 + 9*h0*h1^2 + 3*h0^2*h2 + 2*h1^3 + 6*h0*h1*h2, h0^2*h1 + 4*h0*h1^2 + 2*h0^2*h2 + h0^2*h3 + 8*h0*h1*h2 + 3*h1^3 + 4*h1^2*h2 + 2*h0*h2^2 + 2*h0*h1*h3, h0*h1^2 + 4*h0*h1*h2 + 2*h1^3 + 7*h1^2*h2 + 2*h0*h1*h3 + 3*h0*h2^2 + 4*h1*h2^2 + 2*h1^2*h3 + 2*h0*h2*h3, h1^3 + 6*h1^2*h2 + 9*h1*h2^2 + 3*h1^2*h3 + 2*h2^3 + 6*h1*h2*h3]}
a(n) = {my(v); v=[1, 1, 0, 0]; for(i=2, n, v=Next(v[1], v[2], v[3], v[4])); v[1]+v[4]+3*(v[2]+v[3])} \\ Andrew Howroyd, Jun 20 2017
(Python)
from itertools import islice
def A288490_gen(): # generator of terms
f, g, h, p = 1, 1, 0, 0
while True:
yield f+3*(g+h)+p
a, b = f+(g<<1), g+(h<<1)
f, g, h, p = a*(f*(a+(b<<1)-h)+g**2), f*(p*a+b*(a+(g<<1))+2*h**2)+g**2*(g+(b<<1)), f*(g*(b+(h<<1))+3*h**2)+g*(g*((b<<1)+3*h)+(h<<1)**2)+p*(f*b+g*a), b*(g*(3*p+b+(h<<1))+h**2)
CROSSREFS
Cf. A297536 (maximum independent vertex sets in the n-Hanoi graph).
Cf. A321249 (maximal independent vertex sets in the n-Hanoi graph).
Cf. A288839 (chromatic polynomials of the n-Hanoi graph).
Cf. A193233 (chromatic polynomial with highest coefficients first).
Cf. A137889 (directed Hamiltonian paths in the n-Hanoi graph).
Cf. A286017 (matchings in the n-Hanoi graph).
Cf. A193136 (spanning trees of the n-Hanoi graph).
Cf. A288796 (undirected paths in the n-Hanoi graph).
KEYWORD
nonn
AUTHOR
Eric W. Weisstein, Jun 16 2017
EXTENSIONS
a(5) from Andrew Howroyd, Jun 19 2017
STATUS
approved