A118691 Semiprimes for which the digital root is also a semiprime.

4, 6, 9, 15, 22, 33, 49, 51, 58, 69, 85, 87, 94, 121, 123, 141, 159, 166, 177, 202, 213, 247, 249, 265, 267, 274, 301, 303, 319, 321, 339, 346, 355, 382, 391, 393, 411, 427, 445, 447, 454, 481, 501, 517, 519, 526, 535, 537, 553, 562, 573, 589, 591, 634, 679

Offset

1,1

Links

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

Example

87 is a semiprime and its digital root 6 is also a semiprime.

Mathematica

Select[Range[680], PrimeOmega[#] == 2 && PrimeOmega[Mod[# - 1, 9] + 1] == 2 &] (* Amiram Eldar, Feb 06 2021 *)

Crossrefs

Cf. A001358, A010888.

Sequence in context: A344468 A116589 A118688 * A087718 A033476 A183978

Adjacent sequences: A118688 A118689 A118690 * A118692 A118693 A118694

Keyword

base,nonn

Author

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

Status

approved