OFFSET
1,1
COMMENTS
This is to pentagonal numbers A000326 as A113940 is to triangular numbers A000217. These may be seen as the 5th and 3rd row of an infinite array of k-gonal numbers which are also brilliant numbers, where the 4th row is A001248 squares of primes. - Jonathan Vos Post, Apr 05 2009
EXAMPLE
a(1) = 35 = 5th pentagonal number = 5*(3*5-1)/2 = 5 * 7, with the two prime factors each being one digit in length. a(2) = 247 = 13th pentagonal number = 13*(3*13-1)/2 = 13 * 19, with the two prime factors each being two digits in length. a(6) = 15251 = 101 * 151, with the two prime factors each being three digits in length. - Jonathan Vos Post, Apr 05 2009
17767 is the 109th pentagonal number and 17767=109*163 is brilliant.
CROSSREFS
KEYWORD
base,nonn
AUTHOR
Giovanni Resta, Jan 31 2006
EXTENSIONS
Edited by N. J. A. Sloane, Apr 07 2009 at the suggestion of R. J. Mathar
Two more terms from R. J. Mathar, Apr 06 2009
STATUS
approved