%I A140661
%S A140661 2,3,5,6,7,8,11,13,14,15,17,18,19,20,24,25,28,29,31,32,33,34,38,40,41,
%T A140661 44,46,47,48,49,54,55,56,57,62,63,64,65,68,69,70,71,73,75,76,77,83,85,
%U A140661 89,90,92,93,100,101,104,105,106,107,109,110,111,113,119,120,121,122
%N A140661 Number of pairs (b,c) with the same prime factors, 1<=b<=c<=n.
%C A140661 If pairs are restricted to b<c, we get the variant 0, 0, 1, 1, 1, 1, 
               3, 4, 4, 4, 5, 5, 5, 5, 8, 8, 10, 10, 11, 11, 11, 11, 14, 15,...
%H A140661 P. Erdos and T. Motzkin, <a href="http://www.jstor.org/stable/2324351">
               Density of pairs with same prime factors</a>, Am. Math. Month. vol 
               97 no 10 (1990) p 937, problem 5735.
%e A140661 a(16)=24 counts the 16 pairs (b,b) with 1<=b<=16 plus the 8 pairs (2,
               4), (2,8), (2,16), (4,8), (4,16), (8,16), (3,9), (6,12).
%Y A140661 Sequence in context: A028778 A094180 A116545 this_sequence A063966 A123030 
               A063752
%Y A140661 Adjacent sequences: A140658 A140659 A140660 this_sequence A140662 A140663 
               A140664
%K A140661 easy,nonn
%O A140661 2,1
%A A140661 R. J. Mathar (mathar(AT)strw.leidenuniv.nl), Jul 11 2008