A118692 Semiprimes for which the multiplicative digital root is also a semiprime.

4, 6, 9, 14, 22, 33, 39, 74, 82, 86, 91, 93, 119, 122, 123, 133, 141, 161, 213, 217, 218, 221, 249, 267, 289, 298, 319, 321, 346, 377, 382, 386, 391, 411, 417, 422, 437, 466, 471, 473, 478, 481, 489, 611, 626, 634, 662, 674, 679, 681, 697, 721, 734, 737, 746

Offset

1,1

Links

Amiram Eldar, Table of n, a(n) for n = 1..10000

Example

39 is a semiprime and its multiplicative digital root 4 is also a semiprime.

Mathematica

pod[n_] := Times @@ IntegerDigits[n]; multDigRoot[n_] := NestWhile[pod, n, # > 9 &]; Select[Range[1000], PrimeOmega[#] == 2 && PrimeOmega[multDigRoot[#]] == 2 &] (* Amiram Eldar, Feb 06 2021 *)

Crossrefs

Cf. A001358, A031347.

Sequence in context: A257559 A118689 A364489 * A020706 A226271 A137371

Adjacent sequences: A118689 A118690 A118691 * A118693 A118694 A118695

Keyword

base,nonn

Author

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

Status

approved