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URL: https://oeis.org/A327240

⇱ A327240 - OEIS

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A327240
3-smooth numbers k such that k-1 and (k-2)/2 are prime.
3
6, 8, 12, 24, 48, 108, 384, 864, 8748, 995328, 2348273369088, 7421703487488, 21422803359744, 3470494144278528, 161919374795459002368, 1838129271989302091317248, 2168345519443636233418208968704, 28070062609828769223367060340342784
OFFSET
1,1
COMMENTS
Numbers k of the form 2^a*3^b such that k-1 and (k-2)/2 are prime.
For all terms k except 6 and 8, k-2 is in A325204.
All terms except 6 and 12 end in 4 or 8.
LINKS
Ray Chandler, Table of n, a(n) for n = 1..39 (terms < 10^1000)
EXAMPLE
a(3)=12 is a term because 12=2^2*3 and 11 and 10/2 are prime.
MATHEMATICA
nmax = 10^35;
Select[Sort[Flatten[Table[2^i*3^j, {j, 0, Log[3, nmax]}, {i, Log[2, nmax/3^j]}]]], PrimeQ[# - 1] && PrimeQ[(# - 2)/2] &]
CROSSREFS
KEYWORD
nonn
AUTHOR
Ray Chandler, Sep 14 2019
STATUS
approved