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

1,1

Comments

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

Links

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

Example

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.

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

Prog

(Magma)
IsSemiprime:=func<i|&+[d[2]: d in Factorization(i)] eq 2>;
[nn1: n in [1..100] | IsSemiprime(nn1) where nn1 is Seqint([1] cat Intseq(n) cat Intseq(n))]; // Bruno Berselli, Jan 30 2013

Crossrefs

Cf. A001358, A210511.

Sequence in context: A082945 A316849 A316850 * A084043 A291266 A033284

Adjacent sequences: A210708 A210709 A210710 * A210712 A210713 A210714

Keyword

nonn,base,easy

Author

Jonathan Vos Post, Jan 29 2013

Status

approved