login
Number of unlabeled connected identity interval graphs with n nodes.
(Formerly M3617)
1

%I M3617 #20 Dec 18 2021 20:40:09

%S 1,0,0,0,0,4,27,172,1141,8017,60319,486372,4196384,38621356,377949874,

%T 3920335179,42975606304,496545261764,6031989895262,76867521958187,

%U 1025417405366410,14292224499308110,207763603833695113,3144818752057400765

%N Number of unlabeled connected identity interval graphs with n nodes.

%D P. J. Hanlon, personal communication.

%D R. W. Robinson, Numerical implementation of graph counting algorithms, AGRC Grant, Math. Dept., Univ. Newcastle, Australia, 1980.

%D N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

%H R. W. Robinson, <a href="/A005974/b005974.txt">Table of n, a(n) for n = 1..30</a>

%H Phil Hanlon, <a href="http://dx.doi.org/10.1090/S0002-9947-1982-0662044-8">Counting interval graphs</a>, Trans. Amer. Math. Soc. 272 (1982), no. 2, 383-426.

%o (Sage) def a(n) : return sum(G.is_connected() and G.is_interval() and G.automorphism_group().order()==1 for G in graphs(n)) # _Eric M. Schmidt_, Mar 25 2015

%K nonn,nice

%O 1,6

%A _N. J. A. Sloane_

%E a(21) onwards added by _N. J. A. Sloane_, Oct 19 2006 from the Robinson reference

%E Definition edited by _Eric M. Schmidt_, Mar 25 2015