A066372 - OEIS

A066372

Number of different shapes formed by bending a piece of wire of length n in the plane.

1, 1, 2, 3, 5, 8, 15, 23, 43, 71, 128, 209, 379, 650, 1145, 1928, 3422, 5908, 10295, 17530, 30738, 53088, 91971, 157194, 273621, 471865, 814557, 1393822, 2414895, 4157492, 7160018, 12253782, 21163410, 36381025, 62549316, 107029982, 184430758

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OFFSET

1,3

COMMENTS

Wire is marked into n equal segments by n-1 marks, is bent at right angles at each of these points, making each segment parallel to one of two rectangular axes. (Stays in plane, bends are of +-90 degs.) May cross itself but is not self-coincident over a finite length. Two configurations which differ only in a rotation or turning over are not counted as different.

In addition to not allowing straight segments, there are two further subtle differences between the counting here and the counting in A001997. In this sequence, if the wire effectively forms a closed loop, then that shape is counted only once, whereas in A001997 the position of the ends of the wire matters. Similarly, the same consideration applies to places where the wire is self-coincident. In this sequence, we assume our eyes are not good enough to distinguish which of two (or more) ways of bending the wire achieve the same shape. These distinctions first matter for n=8 where in this sequence all three arrangements which look like a slanted 8 are equivalent. - Sean A. Irvine, Oct 10 2023

REFERENCES

Deborah Freedman, dlf(AT)alumni.princeton.edu, personal communication.

LINKS

Table of n, a(n) for n=1..37.

Erich Friedman, Illustration of initial terms

Ron Knott, Watch Out for Fibonacci Forgeries - Right-Angled Links?

Index entries for sequences obtained by enumerating foldings

EXAMPLE

Let LRUD denote left, right, up, down. Then for n = 1..4 the solutions are R, RD, RDL, RDR, RDLU, RDLD, RDRD.

For n=5 the 5 shapes are:

__.__. __. .__ |__. .__. __.

|__| |__| |__| | |__| |__.