0,2

n(n-3) (n >= 3) is the number of [body] diagonals of an n-gonal prism - Antreas P. Hatzipolakis (xpolakis(AT)otenet.gr).

a(n) = A028387(n)-1. Half of the difference between n(n+1)(n+2)(n+3) and the largest square less than it. Calling this difference "SquareMod": a(n) = (1/2)*SqareMod(n(n+1)(n+2)(n+3)). - Rainer Rosenthal (r.rosenthal(AT)web.de), Sep 04 2004

n != -2 such that x^4 + x^3 - n*x^2 + x + 1 is reducible over the integers. Starting at 10: n such that x^4 + x^3 - n*x^2 + x + 1 is a product of irreducible quadratic factors over the integers. - James Buddenhagen (jbuddenh(AT)gmail.com), Apr 19 2005

If a 3-set Y and a 3-set Z, having two element in common, are subsets of an n-set X then a(n-4) is the number of 3-subests of X intersecting both Y and Z. - Milan R. Janjic (agnus(AT)blic.net), Oct 03 2007

Milan Janjic, Two Enumerative Functions

P. De Geest, Palindromic Quasipronics of the form n(n+x)

a:=n->sum(binomial(n, 1), j=4..n): seq(a(n), n=3..47); - Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Feb 12 2007

a:=n->sum(sum(3, j=3..n)/3, k=0..n): seq(a(n), n=2..46); - Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), May 30 2007

Cf. A028387, A062145.

Adjacent sequences: A028549 A028550 A028551 this_sequence A028553 A028554 A028555

Sequence in context: A048218 A009876 A013921 this_sequence A009877 A009880 A025712

nonn,easy,nice

Patrick De Geest (pdg(AT)worldofnumbers.com)