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A105399
Largest prime <= numbers of the form 6k+3 (duplicates removed).
6
3, 7, 13, 19, 23, 31, 37, 43, 47, 53, 61, 67, 73, 79, 83, 89, 97, 103, 109, 113, 127, 131, 139, 151, 157, 163, 167, 173, 181, 193, 199, 211, 223, 229, 233, 241, 251, 257, 263, 271, 277, 283, 293, 307, 313, 317, 331, 337, 349, 353, 359, 367, 373, 379, 383, 389
OFFSET
1,1
COMMENTS
Apart from the initial 3, the same as A049591. [Proof from T. Khovanova, Jan 23 2008: True for primes up to 5 by inspection. Higher primes must be of the form 6k+1 or 6k+5 since 6k+2 and 6k+4 are divisible by 2 and 6k+3 is divisible by 3. So searching the prime p backwards from the composite, odd 6k+3 in steps of 2 implies that p+2, skipped during that scan, is composite. So p is not in A001359 but in A049591.] - R. J. Mathar, Jan 28 2008
LINKS
EXAMPLE
7 is in the sequence because 7 is the largest prime < 9=6*1+3.
MATHEMATICA
pp[n_] := Block[{k = n}, While[ ! PrimeQ[k], k-- ]; k]; Union[Table[pp[6n + 3], {n, 0, 65}]] (* Ray Chandler, Oct 17 2006 *)
Union[If[PrimeQ[#], #, NextPrime[#, -1]]&/@(6*Range[0, 70]+3)] (* Harvey P. Dale, Aug 20 2021 *)
CROSSREFS
Cf. A106002.
Cf. A049591.
Sequence in context: A291283 A336377 A133387 * A133261 A113911 A365241
KEYWORD
easy,nonn
AUTHOR
Giovanni Teofilatto, May 01 2005
EXTENSIONS
Edited, corrected and extended by Ray Chandler, Oct 17 2006
STATUS
approved