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 A232449 #54 Mar 27 2025 06:11:38
%S A232449 16661,1066601,100666001,10006660001,1000066600001,100000666000001,
%T A232449 10000006660000001,1000000066600000001,100000000666000000001,
%U A232449 10000000006660000000001,1000000000066600000000001,100000000000666000000000001,10000000000006660000000000001,1000000000000066600000000000001
%N A232449 The palindromic Belphegor numbers: (10^(n+3)+666)*10^(n+1)+1.
%C A232449 Though this sequence rarely contains primes (see A232448), most of its members tend to contain a few very large prime factors. The name stems from 'Belphegor's Prime', a(13), which was so named by Clifford Pickover (see link). [Comment corrected by _N. J. A. Sloane_, Dec 14 2015]
%H A232449 Stanislav Sykora, <a href="/A232449/b232449.txt">Table of n, a(n) for n = 0..497</a>
%H A232449 Tony Padilla and Brady Haran, <a href="https://www.youtube.com/watch?v=zk_Q9y_LNzg">The Most Evil Number</a>, Numberphile video (2018)
%H A232449 Clifford A. Pickover, <a href="http://sprott.physics.wisc.edu/pickover/pc/1000000000000066600000000000001.html">Belphegor's Prime: 1000000000000066600000000000001</a>
%H A232449 Simon Singh, <a href="http://www.bbc.co.uk/news/magazine-24724635">Homer Simpson's scary math problems</a>. BBC News. Retrieved 31 October 2013.
%H A232449 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/BelphegorNumber.html">Belphegor Number</a>
%H A232449 Wikipedia, <a href="http://en.wikipedia.org/wiki/Belphegor%27s_Prime">Belphegor's prime</a>
%H A232449 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (111,-1110,1000).
%F A232449 a(n) = 666*10^(n+1)+100^(n+2)+1.
%F A232449 G.f.: (16661 - 782770*x + 767000*x^2) / ((1 - x)*(1 - 10*x)*(1 - 100*x)). [_Bruno Berselli_, Nov 25 2013]
%t A232449 A232449[n_] := 100^(n+2) + 666*10^(n+1) + 1; Array[A232449, 15, 0] (* or *)
%t A232449 LinearRecurrence[{111, -1110, 1000}, {16661, 1066601, 100666001}, 15] (* _Paolo Xausa_, Mar 27 2025 *)
%o A232449 (PARI) Belphegor(k)=(10^(k+3)+666)*10^(k+1)+1; nmax = 498; v = vector(nmax); for (n=0,#v-1, v[n+1]=Belphegor(n))
%Y A232449 Cf. A232448, A232450, A232451.
%Y A232449 Subsequence of A118598.
%K A232449 nonn,easy
%O A232449 0,1
%A A232449 _Stanislav Sykora_, Nov 24 2013