A071780 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).

1, 7, 12, 15, 21, 26, 30, 33, 38, 60, 63, 65, 120, 189, 240, 296, 344, 442, 480, 567, 960, 1476, 1701, 1920, 3840, 4448, 5103, 7680, 15309, 15360, 30720, 45927, 57128, 61440, 66944, 122880, 137781, 245760, 413343, 491520, 538448, 983040, 1051136, 1240029

Offset

0,2

Links

Table of n, a(n) for n=0..43.

Prog

(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, ", "))

Crossrefs

Sequence in context: A072834 A138615 A210254 * A353219 A305553 A338103

Adjacent sequences: A071777 A071778 A071779 * A071781 A071782 A071783

Keyword

nonn

Author

Benoit Cloitre, Jun 04 2002

Extensions

More terms from Ryan Propper, Sep 28 2005

a(24)-a(43) from Donovan Johnson, May 30 2010

Status

approved