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 A049479 #66 Feb 16 2025 08:32:40
%S A049479 3,7,3,31,3,127,3,7,3,23,3,8191,3,7,3,131071,3,524287,3,7,3,47,3,31,3,
%T A049479 7,3,233,3,2147483647,3,7,3,31,3,223,3,7,3,13367,3,431,3,7,3,2351,3,
%U A049479 127,3,7,3,6361,3,23,3,7,3,179951,3,2305843009213693951,3,7,3,31
%N A049479 Smallest prime dividing 2^n - 1.
%C A049479 If p is prime then a(p) == 1 (mod p). Are there composite numbers k such that a(k) == 1 (mod k)? - _Thomas Ordowski_, Jan 27 2014
%C A049479 Yes, up to 1200, the following composites have the desired property: 169, 221, 323, 611, 779, 793, 923, 1121, 1159. - _Michel Marcus_, Jan 28 2014
%C A049479 a(n) <= a(lpf(n)) for every n, where lpf(n) = A020639(n). For which n is a(n) < a(lpf(n))? See A236769. - _Thomas Ordowski_, Jan 30 2014
%H A049479 Max Alekseyev, <a href="/A049479/b049479.txt">Table of n, a(n) for n = 2..1236</a>
%H A049479 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/MersenneNumber.html">Mersenne Number</a>.
%F A049479 a(n) > lpf(n) while a(2k) = 3 and a(2k+1) > 2*lpf(2k+1), where lpf(m) = A020639(m). - _Thomas Ordowski_, Jan 29 2014
%F A049479 For k >= 1, a(2k) = 3, a(6k-3)=7. - _Zak Seidov_, Mar 21 2014
%e A049479 a(6)=3 since 2^6 - 1 = 63 = 3^2*7.
%t A049479 a = {}; Do[w = 2^n - 1; c = FactorInteger[w]; b = c[[1]][[1]]; AppendTo[a, b], {n, 2, 65}]; a (* _Artur Jasinski_, Dec 11 2007 *)
%t A049479 FactorInteger[#][[1,1]]&/@(2^Range[2,70]-1) (* _Harvey P. Dale_, Nov 18 2019 *)
%o A049479 (Python)
%o A049479 from sympy import factorint
%o A049479 def A049479(n):
%o A049479 return min(factorint(2**n-1)) # _Chai Wah Wu_, Jun 03 2019
%Y A049479 Cf. A005420.
%K A049479 nonn
%O A049479 2,1
%A A049479 _Olivier Gérard_
%E A049479 More terms from Michael Lugo (mlugo(AT)thelabelguy.com), Dec 22 1999
%E A049479 Terms to a(500) in b-file from _T. D. Noe_, Dec 06 2006
%E A049479 a(501)-a(1060) in b-file from _Michel Marcus_, Sep 15 2017
%E A049479 a(1061)-a(1236) in b-file added at the suggestion of _Eric Chen_ by _Max Alekseyev_, Apr 25 2022