A290985 Number of amicable pairs with lesser member at most 2^n.

0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 2, 3, 5, 7, 8, 9, 16, 22, 30, 42, 57, 68, 95, 133, 167, 205, 257, 342, 460, 605, 786, 1043, 1356, 1749, 2294, 2905, 3757, 4811, 6181, 7893, 10146, 13051, 16762, 21396, 27380, 34939, 44449, 56481, 71453, 90783, 115200, 145702, 184045, 232899, 294326, 372216

Offset: 1

Author

Charles R Greathouse IV, Aug 16 2017

Keywords

• nonn
• hard

Comments

Sergei Chernykh led a BOINC project to find a(64) = 2390655. The project took 7 months and used several hundred CPU-years of computation time.

Links

• Amiram Eldar, Table of n, a(n) for n = 1..66 (calculated from Chernykh's database)
• Sergei Chernykh, Amicable Numbers.

Crossrefs

Cf. A002025, A066873, A259180.

Programs

PARI

has(n,f=factor(n))=my(t=sigma(f)-n); t>n && sigma(t)==n+t
a(n)=my(s); forfactored(k=1,2^n, if(has(k[1],k[2]), s++)); s