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 A000703 M3265 N1318 #32 Feb 16 2025 08:32:21 %S A000703 4,6,7,7,8,9,9,10,10,10,11,11,12,12,12,13,13,13,13,14,14,14,15,15,15, %T A000703 15,16,16,16,16,16,17,17,17,17,18,18,18,18,18,19,19,19,19,19,19,20,20, %U A000703 20,20,20,21,21,21,21,21,21,22,22,22,22,22,22,22,23,23,23,23,23,23,24,24,24,24 %N A000703 Chromatic number (or Heawood number) of nonorientable surface with n crosscaps. %D A000703 J. L. Gross and T. W. Tucker, Topological Graph Theory, Wiley, 1987; see Table 5.2 p. 221. %D A000703 J. L. Gross and J. Yellen, eds., Handbook of Graph Theory, CRC Press, 2004; p. 368 and 631. %D A000703 N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence). %D A000703 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). %H A000703 T. D. Noe, <a href="/A000703/b000703.txt">Table of n, a(n) for n = 0..1000</a> %H A000703 K. Appel and W. Haken, <a href="http://projecteuclid.org/euclid.ijm/1256049011">Every planar map is four colorable. I. Discharging</a>, Illinois J. Math. 21 (1977), no. 3, 429-490. %H A000703 G. A. Dirac, <a href="http://dx.doi.org/10.4153/CJM-1952-043-9">Map-color theorems</a>, Canad. J. Math., 4 (1952), 480ff. %H A000703 G. Ringel & J. W. T. Youngs, <a href="http://www.pnas.org/content/60/2/438.full.pdf">Solution Of The Heawood Map-Coloring Problem</a>, Proc. Nat. Acad. Sci. USA, 60 (1968), 438-445. %H A000703 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/ChromaticNumber.html">Chromatic Number</a> %H A000703 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/HeawoodConjecture.html">Heawood Conjecture</a> %F A000703 a(n) = floor((7+sqrt(1+24*n))/2). %p A000703 A000703:=n->floor((7+sqrt(1+24*n))/2): seq(A000703(n), n=0..150); # _Wesley Ivan Hurt_, Apr 24 2017 %t A000703 Floor[(7+Sqrt[1+24*Range[0,80]])/2] (* _Harvey P. Dale_, Feb 03 2012 *) %o A000703 (Haskell) %o A000703 a000703 = floor . (/ 2) . (+ 7) . sqrt . (+ 1) . (* 24) . fromInteger %o A000703 -- _Reinhard Zumkeller_, Dec 04 2012 %Y A000703 Cf. A000934 (the orientable case). %K A000703 nonn,nice,easy %O A000703 0,1 %A A000703 _N. J. A. Sloane_