A165972 Nonprimes k such that the sum of the smallest and largest divisor of k is prime.

1, 4, 6, 10, 12, 16, 18, 22, 28, 30, 36, 40, 42, 46, 52, 58, 60, 66, 70, 72, 78, 82, 88, 96, 100, 102, 106, 108, 112, 126, 130, 136, 138, 148, 150, 156, 162, 166, 172, 178, 180, 190, 192, 196, 198, 210, 222, 226, 228, 232, 238, 240, 250, 256, 262, 268, 270, 276

Offset: 1

Author

Juri-Stepan Gerasimov, Oct 02 2009, Oct 14 2009

Keywords

• nonn
• less

Comments

n-(-1)^n is prime.

Largest nonprimeJuri-Stepan Gerasimov, Oct 29 2009]

Examples

			k=1 is in the sequence because 1+1=2 is prime. k=4 is in the sequence because 1+4=5 is prime.
		

Crossrefs

Cf. A000040, A006093, A018252.

Programs

Mathematica

Select[Range[300],!PrimeQ[#]&&With[{d=Divisors[#]},PrimeQ[d[[1]]+ d[[-1]]]]&] (* Harvey P. Dale, Oct 01 2015 *)

Formula

A006093 \ {2}.

Extensions

Entries checked by R. J. Mathar, Oct 10 2009