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A201220 Numbers m such that m, m-1, m-2 and m-3 are 1,2,3,4-almost primes respectively. 1
107, 263, 347, 479, 863, 887, 1019, 2063, 2447, 3023, 3167, 3623, 5387, 5399, 5879, 6599, 6983, 7079, 8423, 8699, 9743, 9887, 10463, 11807, 12263, 12347, 14207, 15383, 15767, 18959, 20663, 22343, 23039, 23567, 24239, 27239, 32183, 33647, 33767, 37799 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
Following a suggestion of Claudio Meller.
m is of the form 12k-1, so m-2 is a multiple of 3 and m-3 is a multiple of 4.
LINKS
EXAMPLE
6599 is prime, 6598=2*3299 is semiprime, 6597=3*3*733 is 3-almost prime, 6596=2*2*17*97 is 4-almost prime.
MATHEMATICA
primeCount[n_] := Plus @@ Transpose[FactorInteger[n]][[2]]; Select[Range[40000], primeCount[#] == 1 && primeCount[#-1] == 2 && primeCount[#-2] == 3 && primeCount[#-3] == 4 &] (* T. D. Noe, Nov 28 2011 *)
Select[Range[40000], PrimeOmega[Range[#, #+3]]=={4, 3, 2, 1}&]+3 (* Harvey P. Dale, Dec 10 2011 *)
SequencePosition[PrimeOmega[Range[40000]], {4, 3, 2, 1}][[;; , 2]] (* Harvey P. Dale, Oct 08 2023 *)
PROG
(PARI) list(lim)=my(v=List(), L=(lim-2)\3, t); forprime(p=3, L\3, forprime(q=3, min(p, L\p), t=3*p*q+2; if(isprime(t) && isprime((t-1)/2) && bigomega(t-3)==4, listput(v, t)))); Set(v) \\ Charles R Greathouse IV, Feb 02 2017
CROSSREFS
Subsequence of A005385 and of A201147.
Sequence in context: A142914 A089635 A248402 * A088563 A142222 A123300
KEYWORD
nonn
AUTHOR
Antonio Roldán, Nov 28 2011
STATUS
approved

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Last modified February 29 23:21 EST 2024. Contains 370428 sequences. (Running on oeis4.)