A106551 a(n) = n-th semiprime + (concatenation of its two divisors, largest divisor first).

26, 38, 42, 62, 86, 68, 94, 134, 80, 158, 146, 206, 110, 230, 172, 278, 126, 224, 170, 250, 350, 374, 200, 302, 446, 194, 494, 260, 518, 380, 228, 406, 566, 290, 638, 484, 350, 710, 296, 1232, 734, 536, 562, 330, 806, 614, 854, 1454, 440, 878, 470, 950, 692

Offset

1,1

Comments

Concatenation of the divisors starting with the smallest one leads to a different sequence.

Links

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

Example

First semiprime is 4; 4 is 2*2; 26=4+22.
Second semiprime is 6; 6 is 3*2; 38=6+32.
Third semiprime is 9; 9 is 3*3; 42=9+33.
Fourth semiprime is 10; 10 is 5*2; 62=10+52.

Maple

N:= 1000: # for terms <= N
Primes:= select(isprime, [2, seq(i, i=3..N/2, 2)]):
R:= NULL:
for i from 1 to nops(Primes) do for j from 1 to i while Primes[i]*Primes[j] <= N do
  R:= R, [Primes[i]*Primes[j], Primes[i]*10^(1+ilog10(Primes[j]))+Primes[j]]
od od:
map(convert, sort([R], (x, y) -> x[1]<y[1]), `+`); # Robert Israel, Jun 09 2020

Mathematica

cp[n_] := Block[{p = Reverse[ First /@ FactorInteger[ n]]}, If[ Length[p] == 1, p = Join[p, p]]; n + FromDigits[ Join @@ IntegerDigits@p]]; cp /@ Select[ Range@ 160, 2 == PrimeOmega@ # &] (* Giovanni Resta, Jun 10 2020 *)

Crossrefs

Sequence in context: A213012 A171745 A045092 * A106553 A371696 A178662

Adjacent sequences: A106548 A106549 A106550 * A106552 A106553 A106554

Keyword

base,easy,nonn,look

Author

Eric Angelini, May 09 2005

Extensions

Edited by N. J. A. Sloane, Sep 29 2007

Corrected and extended by Harvey P. Dale, Jun 03 2012

a(40) and a(48) corrected by Robert Israel, Jun 09 2020

Status

approved