%I A038721
%S A038721 2,18,110,570,2702,12138,52670,223290,931502,3842058,15718430,63928410,
%T A038721 258885902,1045076778,4208939390,16921719930,67944897902,272553908298,
%U A038721 1092539107550,4377127901850,17529428119502,70180466208618
%N A038721 k=2 column of A038719.
%C A038721 For n>=1, a(n) is equal to the number of functions f: {1,2,...,n+1}->
               {1,2,3,4} such that Im(f) contains 2 fixed elements. - Aleksandar 
               M. Janjic and Milan R. Janjic (agnus(AT)blic.net), Feb 27 2007
%C A038721 Let P(A) be the power set of an n-element set A and R be a relation on 
               P(A) such that for all x, y of P(A), xRy if x is not a subset of 
               y and y is not a subset of x. Then a(n+1) = |R|. [From Ross La Haye 
               (rlahaye(AT)new.rr.com), Mar 19 2009]
%D A038721 R. B. Nelsen and H. Schmidt, Jr., Chains in power sets, Math. Mag., 64 
               (1991), 23-31.
%D A038721 Ross La Haye, Binary Relations on the Power Set of an n-Element Set, 
               Journal of Integer Sequences, Vol. 12 (2009), Article 09.2.6. [From 
               Ross La Haye (rlahaye(AT)new.rr.com), Mar 19 2009]
%H A038721 Milan Janjic, <a href="http://www.pmfbl.org/janjic/">Enumerative Formulas 
               for Some Functions on Finite Sets</a>
%H A038721 <a href="Sindx_Pos.html#posets">Index entries for sequences related to 
               posets</a>
%F A038721 4^(n+1) - 2*3^(n+1) + 2^(n+1).
%Y A038721 Cf. A038720.
%Y A038721 Sequence in context: A101570 A006043 A112328 this_sequence A064837 A027433 
               A153338
%Y A038721 Adjacent sequences: A038718 A038719 A038720 this_sequence A038722 A038723 
               A038724
%K A038721 nonn,easy
%O A038721 1,1
%A A038721 N. J. A. Sloane (njas(AT)research.att.com), May 02 2000
%E A038721 More terms from Larry Reeves (larryr(AT)acm.org), May 09 2000