A095912 Variant of the pay-phone sequence A095236. Here a slot at the end of the row is always preferred over a slot sandwiched immediately between two used slots.

1, 2, 4, 6, 12, 28, 104, 152, 528, 2208, 9120, 23616, 130944, 278784, 1635840, 14181120, 32186880, 116674560, 1262039040, 2443714560, 58920099840, 161981890560, 1416311930880, 7700720025600, 120779469619200

Offset

1,2

Links

Max Alekseyev, Table of n, a(n) for n = 1..100

Max A. Alekseyev, Enumeration of Payphone Permutations, arXiv:2304.04324 [math.CO], 2023.

Simon Wundling, About a combinatorial problem with n seats and n people, arXiv:2303.18175 [math.CO], 2023. (German)

Example

Example: there are 5 payphones. First arrival may choose any; he selects phone #2. Next arrival must take the furthest away, #5. Next arrival must take either of #3 or #4 (since both have a neighbor on one side and a vacant slot on the other); he chooses #3. Next arrival must take #1 (because end slots are preferred over "sandwiched" slots), leaving #4 for the last arrival. The permutation (25314) is one of a(5)=10 that satisfy the requirements.

Crossrefs

Cf. A095236, A095239, A095240, A095923.

Sequence in context: A177905 A118405 A332345 * A143474 A261341 A220423

Adjacent sequences: A095909 A095910 A095911 * A095913 A095914 A095915

Keyword

nonn

Author

Jon Wild, Jul 13 2004

Extensions

Corrected and extended by Don Reble, Jul 15 2004

Status

approved