OFFSET
1,2
LINKS
Andrew Howroyd, Table of n, a(n) for n = 1..100
Eric Weisstein's World of Mathematics, Chromatic Invariant
Eric Weisstein's World of Mathematics, Path Complement Graph
FORMULA
a(n) = Sum_{k=0..floor(n/2)} binomial(n-k,k)*(-1)^k*(n-k-2)! for n > 3. - Andrew Howroyd, Apr 22 2018
a(n) = ((2*n + 3)*a(n + 1) - (n + 5)*a(n + 2) + a(n + 3))/n for n > 2. - Eric W. Weisstein, Apr 23 2018
a(n) ~ exp(-1) * n! / n^2. - Vaclav Kotesovec, Apr 23 2018
MATHEMATICA
Join[{1, 2}, Table[Sum[Binomial[n - k, k] (-1)^k (n - k - 2)!, {k, 0, Floor[n/2]}], {n, 3, 20}]]
Join[{1, 2}, Table[Gamma[n - 1] HypergeometricPFQ[{1/2 - n/2, -n/2}, {2 - n, -n}, -4], {n, 3, 20}]]
Join[{1, 2}, RecurrenceTable[{-n a[n] + (3 + 2 n) a[1 + n] + (-5 - n) a[2 + n] + a[3 + n] == 0, a[1] == -1, a[2] == 0, a[3] == 1}, a, {n, 20}]]
PROG
(PARI) a(n)={if(n<4, [1, 2, -1][n], sum(k=0, n\2, binomial(n-k, k)*(-1)^k*(n-k-2)!))} \\ Andrew Howroyd, Apr 22 2018
CROSSREFS
KEYWORD
sign
AUTHOR
Eric W. Weisstein, Apr 16 2018
EXTENSIONS
Terms a(17) and beyond from Andrew Howroyd, Apr 22 2018
STATUS
approved