A102782 Repunit semiprimes.

111, 1111, 11111, 1111111, 11111111111, 11111111111111111, 11111111111111111111111111111111111111111111111, 11111111111111111111111111111111111111111111111111111111111

Offset: 1

Author

Shyam Sunder Gupta, Feb 11 2005

Keywords

• base
• nonn

Examples

			a(2)=1111 because 1111=11*101, so 1111 is semiprime as well as a repunit number.
		

Crossrefs

Cf. A046413 the repunit of length n has exactly 2 prime factors.

Programs

Mathematica

Select[Table[FromDigits[PadRight[{},n,1]],{n,60}],PrimeOmega[#]==2&] (* Harvey P. Dale, Aug 28 2013 *)

Formula

a(n) = A000042(A046413(n-1)). - Ray Chandler, Sep 06 2005