A330441 Semiprimes p*q such that the concatenations of p and q in both orders are prime.

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

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