%I A006961 M2584
%S A006961 1,1,3,6,15,31,75,164,388,887,2092,4884,11599,27443,65509,156427,375263
%N A006961 Mappings from n points to themselves with in-degree <= 2.
%D A006961 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, 
               Academic Press, 1995 (includes this sequence).
%D A006961 N. G. de Bruijn and D. A. Klarner, Multisets of aperiodic cycles, SIAM 
               J, Algeb. Discrete Meth., 3 (1982), 359-368.
%Y A006961 Cf. A006961.
%Y A006961 Sequence in context: A034739 A097941 A117161 this_sequence A034740 A152167 
               A105476
%Y A006961 Adjacent sequences: A006958 A006959 A006960 this_sequence A006962 A006963 
               A006964
%K A006961 nonn,easy,nice
%O A006961 0,3
%A A006961 Simon Plouffe (simon.plouffe(AT)gmail.com)