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A242274
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Numbers k such that k*3^k - 1 is semiprime.
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2
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4, 5, 8, 12, 20, 24, 25, 28, 32, 38, 42, 44, 60, 62, 66, 70, 72, 80, 122, 125, 148, 228, 244, 270, 389, 390, 432, 464, 470, 488, 549, 560, 804, 862
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OFFSET
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1,1
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COMMENTS
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The semiprimes of this form are 323, 1214, 52487, 6377291, 69735688019, 6778308875543, 21182215236074, 640550188738907, 59296646043258911, ...
804 is a term of this sequence. - Luke March, Aug 22 2015
The smallest unresolved value of k is now 862. - Sean A. Irvine, Jun 20 2022
The smallest unresolved value of k is now 866. - Tyler Busby, Oct 06 2023
After the possible term 866, the only remaining 3-digit terms are 912 and 984, unless 920 is a term.
If k is an odd term, then k*3^k - 1 is even, so (k*3^k - 1)/2 is a prime. The next odd terms after 549 are 1125 and 12889. Odd terms are in A366323. (End)
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LINKS
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MATHEMATICA
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Select[Range[241], PrimeOmega[# 3^# - 1]==2&]
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PROG
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(Magma) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [n: n in [2..241] | IsSemiprime(s) where s is n*3^n-1];
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CROSSREFS
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Cf. similar sequence listed in A242273.
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KEYWORD
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nonn,more
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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