A015721 Composite and even n such that phi(n) * sigma(n) is one less than a square.

6, 22, 44, 76, 82, 140, 190, 354, 392, 650, 944, 1072, 1142, 1150, 1668, 2276, 4176, 4192, 4262, 5090, 7260, 8660, 15026, 16064, 22926, 29746, 39436, 42872, 74666, 87498, 94786, 101882, 115772, 118928, 127580, 151348

Offset: 1

Author

Robert G. Wilson v

Keywords

• nonn

Links

• Donovan Johnson, Table of n, a(n) for n = 1..300
• Richard K. Guy, Divisors and desires, Amer. Math. Monthly, 104 (1997), 359-360.

Crossrefs

Subsequence of even terms of A015709.

Programs

PARI

isok(n) = !isprime(n) && !(n%2) && issquare(eulerphi(n)*sigma(n)+1); \\ Michel Marcus, Oct 02 2017

Extensions

Offset corrected by Donovan Johnson, Jan 16 2012