Search: id:A111772
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%I A111772
%S A111772 1,1,3,7,22,77,314
%N A111772 Number of non-isomorphic Average systems with n elements. An Average 
               system has one binary operation "avg" and satisfies the three axioms 
               avg(A,A)=A, avg(A,B)=avg(B,A), avg(avg(A,B),avg(C,D)) = avg(avg(A,
               C),avg(B,D)).
%C A111772 Axiom 1 is idempotence; axiom 2 is commutativity. The only unfamiliar 
               axiom is the third one, mid-quarter-swap, a kind of tree-editing 
               axiom. Together with commutativity, it allows free permutation of 
               nodes at each specific level of a binary tree representing an expression.
%C A111772 The Average axioms are also satisfied by lower semi-lattices, aka idempotent 
               commutative semigroups, by finite Abelian groups with an odd number 
               of elements and by hybrids of these two types.
%D A111772 Richard Schroeppel, Posting to Math-Fun Mailing List, May 01, 2005.
%e A111772 Summary table:
%e A111772 n.Systems...Tables....Group orders
%e A111772 1.......1........1....1
%e A111772 2.......1........2....1
%e A111772 3.......3.......10....1 2 6
%e A111772 4.......7.......92....1.2 2.3 6.2
%e A111772 5......22.....1321....1.5 2.10 4 6.4 20 24
%e A111772 6......77....27882....1.19 2.31 4.7 6.12 12.4 20 24.2 120
%e A111772 7.....314...819330....1.85 2.122 4.32 6.36 8.4 12.19 20.2 24.6 36.2 42 
               48 72 120.2 720
%e A111772 n is the size of the system.
%e A111772 Systems is the count of non-isomorphic systems of that size.
%e A111772 Tables is the total number of tables, with no culling for isomorphism.
%e A111772 Group orders is the number of systems with each size of automorphism 
               group.
%e A111772 For example, there are 314 non-isomorphic Average systems with 7 elements.
%e A111772 85 of those systems have the trivial automorphism group (only the identity),
%e A111772 and each system gives rise to 7! = 5040 distinct tables. There's one
%e A111772 system with an automorphism group of 720 elements, which gives rise to 
               only
%e A111772 5040/720 = 7 different tables. The total number of possible 7-element 
               tables
%e A111772 is 7^49, of which roughly 7^7 satisfy the Average rules.
%e A111772 We have the obvious identities 314 = 85 + 122 + 32 + ... + 1 + 2 + 1 
               and 819330 = 5040 * (85/1 + 122/2 + 32/4 + ... + 1/72 + 2/120 + 1/
               720).
%Y A111772 Cf. A111773 (total number).
%Y A111772 Sequence in context: A148688 A075214 A070766 this_sequence A018190 A000228 
               A108070
%Y A111772 Adjacent sequences: A111769 A111770 A111771 this_sequence A111773 A111774 
               A111775
%K A111772 nonn,nice
%O A111772 1,3
%A A111772 N. J. A. Sloane (njas(AT)research.att.com), Nov 21 2005

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