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A259026 Numbers n such that n*23^n-1 is semiprime. 0
1, 2, 6, 8, 18, 21, 24, 26, 28, 30, 33, 230 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

After 290, which may or may not be in the sequence, 420 and 450 are definitely in the sequence.

LINKS

Table of n, a(n) for n=1..12.

K. O'Bryant, Sets of Natural Numbers with Proscribed Subsets, J. Int. Seq. 18 (2015) # 15.7.7

MATHEMATICA

Select[Range[200], PrimeOmega[# 23^# - 1]==2&]

PROG

(MAGMA) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [n: n in [2..200] | IsSemiprime(s) where s is n*23^n-1];

(PARI) isok(n)=bigomega(n*23^n-1)==2 \\ Anders Hellström, Aug 20 2015

CROSSREFS

Similar sequences listed in A242273.

Sequence in context: A053355 A233572 A005823 * A178758 A024431 A152598

Adjacent sequences:  A259023 A259024 A259025 * A259027 A259028 A259029

KEYWORD

nonn,more

AUTHOR

Luke March, Aug 06 2015

STATUS

approved

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Last modified October 19 08:46 EDT 2021. Contains 348074 sequences. (Running on oeis4.)