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A275549 Number of classes of endofunctions of [n] under reversal. 17
1, 1, 3, 18, 136, 1625, 23436, 412972, 8390656, 193739769, 5000050000, 142656721086, 4458051717120, 151437584670385, 5556003465485760, 218946946471875000, 9223372039002259456, 413620131002462320337, 19673204037747448432896, 989209827833222327690890 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

COMMENTS

f and g are in the same class if function g(i) = f(n+1-i) for all i.

Decomposition by class size

.

n    1      2

---------------

1    1      0

2    2      1

3    9      9

4    16     120

5    125    1500

6    216    23220

7    2401   410571

.

Demonstration for the formula: the classes are either of size 1 or 2.

The classes of size 1 is for functions invariant by reversal. They are specified by half their values, including one more if n is odd. Their number is n^(ceiling(n/2)).

So the number of classes under this symmetry is half (the number of functions + the number of classes of size 1).

a(n) is the number of unoriented length n strings with a maximum of n colors. - Andrew Howroyd, Sep 13 2019

LINKS

Andrew Howroyd, Table of n, a(n) for n = 0..200

FORMULA

a(n) = (n^n+n^ceiling(n/2))/2.

PROG

(PARI) a(n) = {(n^n + n^((n+1)\2))/2} \\ Andrew Howroyd, Sep 13 2019

CROSSREFS

Main diagonal of A277504.

Cf. A000312 All endofunctions

Cf. A000169 Classes under translation mod n

Cf. A001700 Classes under sort

Cf. A056665 Classes under rotation

Cf. A168658 Classes under complement to n+1

Cf. A130293 Classes under translation and rotation

Cf. A081721 Classes under rotation and reversal

Cf. A275550 Classes under reversal and complement

Cf. A275551 Classes under translation and reversal

Cf. A275552 Classes under translation and complement

Cf. A275553 Classes under translation, complement and reversal

Cf. A275554 Classes under translation, rotation and complement

Cf. A275555 Classes under translation, rotation and reversal

Cf. A275556 Classes under translation, rotation, complement and reversal

Cf. A275557 Classes under rotation and complement

Cf. A275558 Classes under rotation, complement and reversal

Cf. A078707 Endofunctions symmetric around their middle (stable by reversal).

Sequence in context: A247452 A118970 A003122 * A039618 A183363 A216492

Adjacent sequences:  A275546 A275547 A275548 * A275550 A275551 A275552

KEYWORD

nonn,easy

AUTHOR

Olivier Gérard, Aug 01 2016

STATUS

approved

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Last modified August 9 18:32 EDT 2020. Contains 336326 sequences. (Running on oeis4.)