%I A131651
%S A131651 15,21,27,30,33,35,39,42,51,54,55,57,60,65,66,69,70,77,78,84,85,87,91,
%T A131651 93,95,102,108,110,111,114,115,119,120,123,125,129,130,132,133,138,140,
%U A131651 141,143,145,154,155,156,159,161,168,170,174,177,182,183,185,186,187
%N A131651 Positive integers obtained as the difference of two triangular numbers 
               in exactly 4 ways.
%C A131651 It appears that terms of the sequence are all given by a power of 2 times 
               the cube of an odd prime or a power of 2 times a product of two distinct 
               odd primes. (This has been verified for a(n)<=10000.)
%Y A131651 Cf. A001227, A038550, A072502.
%Y A131651 Sequence in context: A057489 A070811 A138594 this_sequence A134642 A072974 
               A082686
%Y A131651 Adjacent sequences: A131648 A131649 A131650 this_sequence A131652 A131653 
               A131654
%K A131651 nonn
%O A131651 1,1
%A A131651 John W. Layman (layman(AT)math.vt.edu), Sep 10 2007