A118697 - OEIS

%I #6 Mar 07 2024 08:52:19

%S 4,6,9,10,21,34,65,87,123,321,543,789,901,1234,4321,8765,34567,56789,

%T 98765,654321,876543,901234,1234567,7654321,7890123,10987654,21098765,

%U 567890123,12345678901,54321098765,76543210987,87654321098

%N Semiprimes with consecutive digits.

%C Digits can be in ascending or descending order. After 9 comes 0.

%C The number of semiprimes with n consecutive digits: 3,5,5,3,3,3,3,2,1,0,4,0,2,0,2,1,1,1,1,0,1,2,0,1,0,0,2,1,0,0,1,1,1,0,0,0,0,2,1,0,1,0,0,0,2,1,0,0,0,0,..., . - _Robert G. Wilson v_, Jun 16 2006

%t spQ[n_] := Plus @@ Last /@ FactorInteger@n == 2; f[n_] := Block[{u = Range@n, d = Reverse@ Range@n, t = Table[1, {n}]}, Select[ Drop[ Union@ Flatten@ Table[ FromDigits /@ Mod[{u, d} + {i*t, i*t}, 10], {i, 10}], 2], spQ@# &]]; Array[f, 13] // Flatten (* _Robert G. Wilson v_, Jun 16 2006 *)

%Y Cf. A001358.

%K base,nonn

%O 1,1

%A Luc Stevens (lms022(AT)yahoo.com), May 20 2006

%E More terms from _Robert G. Wilson v_, Jun 16 2006