A015709 Composite n such that phi(n) * sigma(n) is one less than a square.

6, 22, 33, 44, 69, 76, 82, 140, 190, 295, 354, 392, 423, 451, 531, 581, 650, 944, 1072, 1119, 1142, 1150, 1668, 2276, 2893, 4043, 4163, 4176, 4192, 4257, 4262, 4509, 5090, 5975, 7260, 8660, 12977, 14927, 15026, 15753, 16064, 19105

Offset: 1

Author

Robert G. Wilson v

Keywords

• nonn

Links

• Donovan Johnson, Table of n, a(n) for n = 1..500
• Richard K. Guy, Divisors and desires, Amer. Math. Monthly, 104 (1997), 359-360.

Programs

PARI

isok(n) = !isprime(n) && issquare(eulerphi(n)*sigma(n)+1); \\ Michel Marcus, Oct 02 2017

Extensions

Offset corrected by Donovan Johnson, Apr 02 2012