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A124677 Minimal total number of multiplications by single letters needed to generate all words of length n in the free monoid on two generators. 2
0, 2, 6, 13, 27 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

LINKS

Table of n, a(n) for n=0..4.

EXAMPLE

Form a tree with the empty word 0 as the root. Each node has potentially 4 children, corresponding to premultiplication by x or y and postmultiplication by x and y.

Layers 0 through 3 of the tree are as follows (the edges, which just join one layer to the next, have been omitted):

.............0.................

.......x...........y...........

..xx.....xy.....yx....yy.......

xxx xxy xyx yxx xyy yxy yyx yyy

a(n) is the minimal number of edges in a subtree that includes the root and all 2^n nodes at level n.

a(3) = 13 because each of xxx,xxy,xyx,xyy,yxx,yxy,yyx,yyy can be obtained in one step from xx,xy,yy; that is, we don't need yx. The corresponding subtree has 2 + 3 + 8 = 13 edges.

a(4) = 27 because one computes successively: 0, x,y, xx,xy,yy, xxx,xyx,xxy,yxy,yyx,yyy and then all 16 words of length 4.

CROSSREFS

See A075099, A075100 for a different way of counting multiplications. Here we only grow the words one letter at a time.

Sequence in context: A254821 A192953 A275970 * A034465 A182614 A055243

Adjacent sequences:  A124674 A124675 A124676 * A124678 A124679 A124680

KEYWORD

more,nonn

AUTHOR

N. J. A. Sloane, Dec 25 2006

EXTENSIONS

Definition clarified by Benoit Jubin, Jan 24 2009

STATUS

approved

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Last modified May 26 20:43 EDT 2017. Contains 287129 sequences.