OFFSET
1,1
COMMENTS
All divisors of 10*T(n)+1 are congruent to 1 or -1 modulo 10; that is, they end in the decimal digit 1 or 9.
LINKS
Harvey P. Dale, Table of n, a(n) for n = 1..1000
Nick Hobson, Triangular Numbers.
EXAMPLE
10*T(9) + 1 = 5*9*10 + 1 = 451 = 11*41, so a(9) = 11.
MATHEMATICA
FactorInteger[#][[1, 1]]&/@(10*Accumulate[Range[60]]+1) (* Harvey P. Dale, Dec 12 2011 *)
PROG
(PARI) a(n) = if(n<1, 0, factor(5*n*(n+1)+1)[1, 1])
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Nick Hobson, Nov 25 2006
STATUS
approved