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A293492 a(n) is the number of sequences (s_1, ..., s_n) of positive integers such that Product_{k=1..n} [s_k, -1/1, 0]^2 = [-1, 0/0, -1]. 0
0, 0, 2, 6, 20, 75, 278 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

COMMENTS

Let M(s) denote the matrix

[s, -1]

[+1, 0]

in SL(2,Z). The we count sequences of positive integers [s_1, ..., s_n] such that Prod_{k=1..n} M(s_k)^2 = - Identity.

This is Problem III in the Ovsienko article.

LINKS

Table of n, a(n) for n=0..6.

Valentin Ovsienko, Partitions of unity in SL(2,Z), negative continued fractions, and dissections of polygons, arXiv:1710.02996 [math.CO], 2017.

CROSSREFS

Sequence in context: A150158 A034010 A135588 * A150159 A150160 A150161

Adjacent sequences:  A293489 A293490 A293491 * A293493 A293494 A293495

KEYWORD

nonn,more

AUTHOR

Michel Marcus, Oct 10 2017

STATUS

approved

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Last modified June 23 09:32 EDT 2018. Contains 305693 sequences. (Running on oeis4.)