cp's OEIS Frontend

This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.

A213457 Intertwining numbers.

Original entry on oeis.org

1, 1, 2, 10, 148, 7384, 1380960, 1058349286
Offset: 1

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Author

N. J. A. Sloane

Keywords

Comments

a(4)=10 for example is the number of ways of arranging 1 a, 2 b's, 3 c's and 4 d's so that if we look at any two letters, i and j say, with i
If the condition is imposed only on pairs of consecutive letters, we get A003121.

Examples

			The 10 sequences for n=4 are dcbadcbdcd dcbadcdbcd dcbdacbdcd dcbdacdbcd dcbdcabdcd dcbdcadbcd dcbdcdabcd dcdbacdbcd dcdbcadbcd dcdbcdabcd.
For example in dcbdacdbcd we see
..ba...b..
.cb..c.bc.
d.bd..db.d
dc.d.cd.cd

References

  • C. L. Mallows, Letter to N. J. A. Sloane, Nov 11 1980
  • N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

Links

Crossrefs

Cf. A003121.

Extensions

Entry revised and given a new A-number by N. J. A. Sloane, Jun 13 2012. The old entry was A004065
a(7) from David W. Wilson, Dec 11 1999
Definition clarified by David Applegate, Jun 14 2012
a(8) from Georg Fischer, Mar 21 2018

Started in 1964 by Neil J. A. Sloane | Maintained by The OEIS Foundation Inc.

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