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## Identifying a Sequence: a Sequence From a Chemical Journal

A chemist reading a technical journal comes across the sequence

1, 3, 7, 22, 82, 333, 1448, ...

and wonders if there is a simple mathematical formula for it.

He or she goes to the main web page of the OEIS and enters the sequence, as in the previous demonstrations.

This produces the following response.

### Greetings from the On-Line Encyclopedia of Integer Sequences!

 A000228 Number of hexagonal polyominoes (or planar polyhexes) with n cells. (Formerly M2682 N1072) 36
 1, 1, 3, 7, 22, 82, 333, 1448, 6572, 30490, 143552, 683101, 3274826, 15796897, 76581875, 372868101, 1822236628, 8934910362, 43939164263, 216651036012, 1070793308942 (list; graph; refs; listen; history; edit; internal format)

Note the keyword "hard"!
This indicates that all the terms known to science are shown - no one knows the next term. No formula, recurrence or simple algorithm is known for this sequence.

Clicking on the link in the line

```Links:     N. J. A. Sloane, Illustration of initial terms.
```
produces the following pictures:

This explains what the sequence is about, and suggests why it is such a hard problem (because of the requirement that the hexagons must lie in a plane).

Click the single right arrow to go to the next demonstration page, or the single left arrow to return to the previous page.

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Last modified December 13 03:51 EST 2018. Contains 318081 sequences. (Running on oeis4.)