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A368238
Semiprimes whose reversal is a prime, ordered by the prime.
1
91, 14, 34, 74, 35, 95, 38, 301, 901, 721, 731, 361, 371, 391, 791, 922, 142, 362, 382, 703, 713, 133, 943, 763, 973, 793, 914, 134, 334, 934, 974, 194, 305, 905, 145, 745, 755, 365, 965, 785, 395, 995, 106, 706, 146, 346, 746, 166, 386, 917, 377, 118, 358, 758, 958, 778, 119, 749, 779, 799, 3101
OFFSET
1,1
LINKS
FORMULA
a(n) = A004086(A085778(n)).
EXAMPLE
a(4) = 74 because A115670(4) = 47 is the 4th prime whose reversal is a semiprime, and 74 is that reversal.
MAPLE
rev:= proc(n) local L, i;
L:= convert(n, base, 10);
add(L[-i]*10^(i-1), i=1..nops(L))
end proc:
map(rev, select(p -> isprime(p) and numtheory:-bigomega(rev(p)) = 2, [seq(i, i=3..1000, 2)]);
MATHEMATICA
s = {}; Do[If[2 == PrimeOmega[sm = FromDigits[Reverse[IntegerDigits[Prime[k]]]]], AppendTo[s, sm]], {k, 200}]; s
CROSSREFS
KEYWORD
nonn,base
AUTHOR
Zak Seidov and Robert Israel, Dec 18 2023
STATUS
approved