0,2

The triangular numbers that are 1/5 are in A077260

Let b(n)=A077260 then a(n)=5*b(n)

G.f.:(-15*x*(x^2+6*x+1))/((x-1)*(x^2-18*x+1)*(x^2+18*x+1)) [From Maksym Voznyy (voznyy(AT)mail.ru), Jul 27 2009]

a(3)=5*990=4950

Cf. A077259, A077260, A077262.

Sequence in context: A058085 A060218 A165892 this_sequence A012507 A143727 A041426

Adjacent sequences: A077258 A077259 A077260 this_sequence A077262 A077263 A077264

easy,nonn

Bruce Corrigan (scentman(AT)myfamily.com), Nov 01 2002