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

Author

Marius A. Burtea, Oct 26 2018

Keywords

• nonn
• base

Examples

			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.
		

Links

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

Crossrefs

Cf. A001358, A100484, A046315, A107342.

Programs

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 *)