%I M3265 N1318 #31 Jun 30 2017 09:09:28
%S 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 15,16,16,16,16,16,17,17,17,17,18,18,18,18,18,19,19,19,19,19,19,20,20,
%U 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 Chromatic number (or Heawood number) of nonorientable surface with n crosscaps.
%D J. L. Gross and T. W. Tucker, Topological Graph Theory, Wiley, 1987; see Table 5.2 p. 221.
%D J. L. Gross and J. Yellen, eds., Handbook of Graph Theory, CRC Press, 2004; p. 368 and 631.
%D N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
%D N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%H T. D. Noe, <a href="/A000703/b000703.txt">Table of n, a(n) for n = 0..1000</a>
%H 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 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 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 Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/ChromaticNumber.html">Chromatic Number</a>
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/HeawoodConjecture.html">Heawood Conjecture</a>
%F a(n) = floor((7+sqrt(1+24*n))/2).
%p A000703:=n->floor((7+sqrt(1+24*n))/2): seq(A000703(n), n=0..150); # _Wesley Ivan Hurt_, Apr 24 2017
%t Floor[(7+Sqrt[1+24*Range[0,80]])/2] (* _Harvey P. Dale_, Feb 03 2012 *)
%o (Haskell)
%o a000703 = floor . (/ 2) . (+ 7) . sqrt . (+ 1) . (* 24) . fromInteger
%o -- _Reinhard Zumkeller_, Dec 04 2012
%Y Cf. A000934 (the orientable case).
%K nonn,nice,easy
%O 0,1
%A _N. J. A. Sloane_