This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A105399 #13 Aug 20 2021 14:11:10
%S A105399 3,7,13,19,23,31,37,43,47,53,61,67,73,79,83,89,97,103,109,113,127,131,
%T A105399 139,151,157,163,167,173,181,193,199,211,223,229,233,241,251,257,263,
%U A105399 271,277,283,293,307,313,317,331,337,349,353,359,367,373,379,383,389
%N A105399 Largest prime <= numbers of the form 6k+3 (duplicates removed).
%C A105399 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
%H A105399 Harvey P. Dale, <a href="/A105399/b105399.txt">Table of n, a(n) for n = 1..1000</a>
%e A105399 7 is in the sequence because 7 is the largest prime < 9=6*1+3.
%t A105399 pp[n_] := Block[{k = n},While[ ! PrimeQ[k], k-- ];k];Union[Table[pp[6n + 3], {n, 0, 65}]] (* _Ray Chandler_, Oct 17 2006 *)
%t A105399 Union[If[PrimeQ[#],#,NextPrime[#,-1]]&/@(6*Range[0,70]+3)] (* _Harvey P. Dale_, Aug 20 2021 *)
%Y A105399 Cf. A106002.
%Y A105399 Cf. A049591.
%K A105399 easy,nonn
%O A105399 1,1
%A A105399 _Giovanni Teofilatto_, May 01 2005
%E A105399 Edited, corrected and extended by _Ray Chandler_, Oct 17 2006