A071780 - OEIS

%I #10 Oct 02 2013 15:12:32

%S 1,7,12,15,21,26,30,33,38,60,63,65,120,189,240,296,344,442,480,567,

%T 960,1476,1701,1920,3840,4448,5103,7680,15309,15360,30720,45927,57128,

%U 61440,66944,122880,137781,245760,413343,491520,538448,983040,1051136,1240029

%N Smallest k > n such that there are exactly n pairs (x,y) (1 <= x <= y <= k) solutions of the equation: phi(xy)=sigma(x)+sigma(y).

%o (PARI) for(n=1,12,s=2; while(sum(i=1,s,sum(j=1,i,if(eulerphi(i*j)-sigma(i)-sigma(j),0,1)))<n,s++); print1(s,","))

%K nonn

%O 0,2

%A _Benoit Cloitre_, Jun 04 2002

%E More terms from _Ryan Propper_, Sep 28 2005

%E a(24)-a(43) from _Donovan Johnson_, May 30 2010