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 A330441 Semiprimes p*q such that the concatenations of p and q in both orders are prime. 1
 21, 33, 51, 93, 111, 133, 177, 201, 219, 247, 253, 327, 411, 427, 573, 589, 679, 687, 763, 793, 813, 889, 993, 1077, 1081, 1119, 1243, 1339, 1347, 1401, 1411, 1497, 1501, 1603, 1623, 1651, 1671, 1821, 1839, 1843, 1851, 1981, 2019, 2047, 2059, 2103, 2157, 2199, 2217, 2469, 2479, 2629, 2761, 2787 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Robert Israel, Table of n, a(n) for n = 1..6933 EXAMPLE a(3)=51 is a member because 51=3*17 and both 317 and 173 are primes. MAPLE N:= 20000: # for all terms <= N catt:= proc(n, m) 10^(1+ilog10(m))*n+m end proc: Primes:= select(isprime, [seq(i, i=3..N/3)]): SP:= [seq(seq([p, q], q = select(`<=`, Primes, min(p, N/p))), p=Primes)]: B:=select(t -> isprime(catt(t[1], t[2])) and isprime(catt(t[2], t[1])), SP): sort(map(convert, B, `*`)); CROSSREFS Cf. A006881. Sequence in context: A141249 A026068 A217263 * A176945 A333857 A070006 Adjacent sequences:  A330438 A330439 A330440 * A330442 A330443 A330444 KEYWORD nonn,base AUTHOR J. M. Bergot and Robert Israel, Dec 14 2019 STATUS approved

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Last modified September 21 07:28 EDT 2021. Contains 347596 sequences. (Running on oeis4.)