A079543 Numbers k such that k has at least two distinct prime factors and if a prime p divides k then (p-1) | (k-1) and (p+1) | (k+1).

74431, 71528191, 125780831, 178708831, 4150390625, 68738591551, 171739186591, 429079903231, 634061169071

Offset: 1

Author

Don Reble, Jan 22 2003

Keywords

• nonn

Examples

			a(1) = 74431 = 7^4 * 31 because 6 and 30 divide 74430 and 8 and 32 divide 74432.
		

Links

• Donovan Johnson, 1384 terms > a(9)

Crossrefs

Intersection of A056729 and A080062, minus A001597.

Programs

Mathematica

Do[ f = Transpose[ FactorInteger[n]][[1]]; If[ Length[f] > 1 && Union[ Mod[n - 1, f - 1]] == {0} && Union[ Mod[n + 1, f + 1]] == {0}, Print[n]], {n, 6, 10^10}]

Extensions

a(6)-a(7) from Donovan Johnson, Apr 09 2010

a(8)-a(9) from Donovan Johnson, Jan 21 2013