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 A003670 M3303 #35 Feb 16 2025 08:32:27
%S A003670 4,7,11,15,18,19,23,25,27,31,32,33,35,41,47,49,55,57,61,63,75,87,89,
%T A003670 91,105,119,121,125,129,133,139,147,153,161,185,189,203,206,213,225,
%U A003670 233,235,241,243,251,257,259,265,273,281,285,293,295,303,305,307,309,317
%N A003670 a(n) is smallest number which is uniquely a(j)+a(k).
%C A003670 Sequence is periodic mod 11301098.
%C A003670 An Ulam-type sequence - see A002858 for many further references, comments, etc. - _T. D. Noe_, Jan 21 2008
%D A003670 Steven R. Finch, Mathematical Constants, Cambridge, 2003, pp. 145-151.
%D A003670 R. K. Guy, "s-Additive sequences", preprint, 1994.
%D A003670 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%H A003670 T. D. Noe, <a href="/A003670/b003670.txt">Table of n, a(n) for n = 1..10000</a>
%H A003670 Steven R. Finch, <a href="http://www.people.fas.harvard.edu/~sfinch/constant/stlrsky/stlrsky.html">Stolarsky-Harborth Constant</a> [Broken link]
%H A003670 Steven R. Finch, <a href="http://web.archive.org/web/20010207195349/http://www.mathsoft.com/asolve/constant/stlrsky/stlrsky.html">Stolarsky-Harborth Constant</a> [From the Wayback machine]
%H A003670 Steven R. Finch, <a href="https://www.emis.de/journals/EM/expmath/volumes/1/1.html">Patterns in 1-additive sequences</a>, Experimental Mathematics 1 (1992), 57-63.
%H A003670 R. K. Guy, <a href="/A007300/a007300.pdf">s-Additive sequences</a>, Preprint, 1994. (Annotated scanned copy)
%H A003670 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/UlamSequence.html">Ulam Sequence</a>
%H A003670 Wikipedia, <a href="http://en.wikipedia.org/wiki/Ulam_number">Ulam number</a>
%H A003670 <a href="/index/U#Ulam_num">Index entries for Ulam numbers</a>
%t A003670 Nest[Append[#, SelectFirst[Union@ Select[Tally@ Map[Total, Select[Permutations[#, {2}], #1 < #2 & @@ # &]], Last@ # == 1 &][[All, 1]], Function[k, FreeQ[#, k]]]] &, {4, 7}, 56] (* _Michael De Vlieger_, Nov 16 2017 *)
%o A003670 (Haskell)
%o A003670 a003670 n = a003670_list !! (n-1)
%o A003670 a003670_list = 4 : 7 : ulam 2 7 a003670_list
%o A003670 -- Function ulam as defined in A002858.
%o A003670 -- _Reinhard Zumkeller_, Nov 03 2011
%K A003670 nonn
%O A003670 1,1
%A A003670 _N. J. A. Sloane_, _Mira Bernstein_