4,2

N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).

J. Riordan, Permutations without 3-sequences, Bull. Amer. Math. Soc., 51 (1945), 745-748.

Jackson, D. M.; Reilly, J. W. Permutations with a prescribed number of $p$-runs. Ars Combinatoria 1 (1976), no. 1, 297-305.

Cf. A047921.

Sequence in context: A012421 A012381 A012376 this_sequence A009552 A002670 A119921

Adjacent sequences: A002627 A002628 A002629 this_sequence A002631 A002632 A002633

nonn

N. J. A. Sloane (njas(AT)research.att.com).