A128686 - OEIS

A128686

Least number of the form semiprime - 1 which is the product of exactly n primes.

3, 9, 8, 24, 32, 64, 128, 864, 1792, 1536, 2048, 4096, 8192, 24576, 73728, 98304, 131072, 393216, 524288, 1048576, 3145728, 6291456, 8388608, 37748736, 50331648, 150994944, 301989888, 268435456, 1207959552, 1610612736, 2147483648, 4294967296, 19327352832, 25769803776, 115964116992, 171798691840, 343597383680, 927712935936, 2886218022912, 1099511627776, 20890720927744, 24189255811072

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OFFSET

1,1

LINKS

Table of n, a(n) for n=1..42.

MATHEMATICA

With[{nn=550000}, {spm1=Select[Range[nn], PrimeOmega[#]==2&]-1}, Table[SelectFirst[{#, PrimeOmega[#]}&/@spm1, #[[2]]==n&], {n, 19}]][[;; , 1]] (* The program generates the first 19 terms of the sequence. *) (* Harvey P. Dale, Jan 01 2026 *)

CROSSREFS

Cf. A005383, A070552, A128665.

Sequence in context: A179589 A368737 A302630 * A161574 A103555 A388464

Adjacent sequences: A128683 A128684 A128685 * A128687 A128688 A128689

KEYWORD

nonn

AUTHOR

Ray Chandler, Mar 20 2007

EXTENSIONS

a(27)-a(42) from Donovan Johnson, Nov 24 2010

STATUS

approved