A210711 Semiprimes formed by concatenating n, n, and 1 for n = 1, 2, 3,....

111, 221, 551, 771, 11111, 14141, 15151, 16161, 19191, 23231, 24241, 29291, 30301, 34341, 36361, 37371, 38381, 39391, 42421, 44441, 47471, 50501, 53531, 55551, 56561, 59591, 62621, 68681, 70701, 74741, 75751, 77771, 79791, 81811, 83831, 84841, 87871, 91911, 95951, 96961, 1001001

Offset: 1

Author

Jonathan Vos Post, Jan 29 2013

Keywords

• nonn
• base
• easy

Comments

This is to A210511 as semiprimes A001358 are to primes A000040.

Examples

			a(1) = 11 because 111 = 3 * 37.
a(2) = 221 because 221 = 13 * 17.
331 is not in the sequence, because it is a prime.
a(5) = 11111 because "11" concatenated with "11" concatenated with "1" = 11111 = 41 * 271.
		

Links

• Robert Israel, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A001358, A210511.

Programs

Magma

IsSemiprime:=func;
[nn1: n in [1..100] | IsSemiprime(nn1) where nn1 is Seqint([1] cat Intseq(n) cat Intseq(n))]; // Bruno Berselli, Jan 30 2013

Maple

read("transforms"):
for n from 1 to 100 do
    L := [n,n,1] ;
    p := digcatL(L) ;
    if numtheory[bigomega](p) = 2 then
        print(p) ;
    end if;
end do: # R. J. Mathar, Jan 30 2013