login
This site is supported by donations to The OEIS Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A201220 Numbers n such that n, n-1, n-2 and n-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.

n is  of the form 12k-1, so n-2 is a multiple of 3 and n-3 is a multiple of 4.

LINKS

Vincenzo Librandi, Table of n, a(n) for n = 1..1700

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 *)

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.

Cf. A005383, A112998, A113000, A113008, A072875, A093552.

Sequence in context: A142914 A089635 A248402 * A088563 A142222 A123300

Adjacent sequences:  A201217 A201218 A201219 * A201221 A201222 A201223

KEYWORD

nonn

AUTHOR

Antonio Roldán, Nov 28 2011

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent | More pages
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy .

Last modified July 25 04:48 EDT 2017. Contains 289779 sequences.