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A106551 a(n) = n-th semiprime + (concatenation of its two divisors, largest divisor first). 1
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 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
Concatenation of the divisors starting with the smallest one leads to a different sequence.
LINKS
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 A178662 A039324
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

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Last modified February 21 10:25 EST 2024. Contains 370228 sequences. (Running on oeis4.)