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A278931 Semiprimes whose ternary representations are also semiprime when read as a decimal number. 1
25, 49, 65, 82, 106, 115, 118, 121, 142, 143, 155, 187, 209, 235, 254, 259, 262, 265, 274, 289, 299, 314, 319, 326, 334, 335, 341, 355, 361, 382, 398, 415, 445, 451, 454, 458, 469, 493, 511, 515, 538, 551, 562, 566, 583, 586, 589, 614, 622, 634, 649, 667, 679 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

K. D. Bajpai, Table of n, a(n) for n = 1..9646

Wikipedia, Ternary numeral system

EXAMPLE

65 is in the sequence because 5*13 = 65 (semiprime) and its ternary representation, 2102 = 2*1051, when read as a decimal number, is also semiprime.

115 is in the sequence because 5*23 = 115 (semiprime) and its ternary representation, 11021 = 103*107, when read as a decimal number, is also semiprime.

MATHEMATICA

Select[Range[5000], PrimeOmega[#] == 2 && PrimeOmega[FromDigits[ IntegerDigits[ #, 3]]] == 2 &]

CROSSREFS

Subsequence of A001358.

Cf. A001358, A005935, A007089, A065721, A089981, A236537.

Sequence in context: A245272 A091300 A112771 * A181782 A322121 A020146

Adjacent sequences:  A278928 A278929 A278930 * A278932 A278933 A278934

KEYWORD

nonn,base

AUTHOR

K. D. Bajpai, Dec 04 2016

STATUS

approved

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Last modified May 15 04:03 EDT 2021. Contains 343909 sequences. (Running on oeis4.)