A321046 Semiprimes for which the concatenation of the digits in the even positions and the concatenation of the digits in the odd positions are semiprimes.

46, 49, 69, 94, 145, 194, 262, 265, 291, 295, 365, 393, 394, 395, 398, 446, 466, 469, 545, 565, 591, 597, 649, 662, 669, 695, 699, 767, 794, 842, 862, 865, 866, 895, 943, 961, 965, 993, 995, 1006, 1046, 1059, 1145, 1154, 1202, 1205, 1241, 1255, 1343, 1345, 1349, 1354, 1355, 1501, 1507, 1541, 1555, 1642, 1649, 1655

Offset

1,1

Links

Marius A. Burtea, Table of n, a(n) for n = 1..22530

Example

46 is a term because 46 = 2*23, 4 = 2*2 and 6 = 2*3 are semiprimes.
469 is a term because 469 = 7*67, 49 = 7*7 and 6 = 2*3 are semiprimes.
1145 is a term because 1145 = 5*229, 14 = 2*7 and 15 = 3*5 are semiprimes.
Also 38159 belongs to the sequence. In fact: 38159 = 11*3469, 319 = 11*29 and 85 = 5*17 are semiprimes.

Mathematica

spQ[n_] := Plus @@ Last /@ FactorInteger[n] == 2; ok[n_] := spQ[n] && Block[{d = IntegerDigits[n]}, If[OddQ@ Length@ d, PrependTo[d, 0]]; AllTrue[ FromDigits /@ Transpose[ Partition[d, 2]], spQ]]; Select[ Range@ 1655, ok] (* Giovanni Resta, Oct 29 2018 *)

Crossrefs

Cf. A001358, A100484, A046315, A107342.

Sequence in context: A181460 A235687 A365009 * A269671 A039354 A043177

Adjacent sequences: A321043 A321044 A321045 * A321047 A321048 A321049

Keyword

nonn,base

Author

Marius A. Burtea, Oct 26 2018

Status

approved