

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.


1



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
(list;
graph;
refs;
listen;
history;
text;
internal format)



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: A098194 A181460 A235687 * A269671 A039354 A043177
Adjacent sequences: A321043 A321044 A321045 * A321047 A321048 A321049


KEYWORD

nonn,base


AUTHOR

Marius A. Burtea, Oct 26 2018


STATUS

approved



