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.
%I A213457 M1988 #36 Jul 09 2025 04:14:03 %S A213457 1,1,2,10,148,7384,1380960,1058349286 %N A213457 Intertwining numbers. %C A213457 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<j, then any pair of i's are separated and surrounded by at least one j. %C A213457 If the condition is imposed only on pairs of consecutive letters, we get A003121. %D A213457 C. L. Mallows, Letter to _N. J. A. Sloane_, Nov 11 1980 %D A213457 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). %H A213457 Georg Fischer, <a href="/A213457/a213457.txt">Perl program</a> %e A213457 The 10 sequences for n=4 are dcbadcbdcd dcbadcdbcd dcbdacbdcd dcbdacdbcd dcbdcabdcd dcbdcadbcd dcbdcdabcd dcdbacdbcd dcdbcadbcd dcdbcdabcd. %e A213457 For example in dcbdacdbcd we see %e A213457 ..ba...b.. %e A213457 .cb..c.bc. %e A213457 d.bd..db.d %e A213457 dc.d.cd.cd %Y A213457 Cf. A003121. %K A213457 nonn,more %O A213457 1,3 %A A213457 _N. J. A. Sloane_ %E A213457 Entry revised and given a new A-number by _N. J. A. Sloane_, Jun 13 2012. The old entry was A004065 %E A213457 a(7) from _David W. Wilson_, Dec 11 1999 %E A213457 Definition clarified by _David Applegate_, Jun 14 2012 %E A213457 a(8) from _Georg Fischer_, Mar 21 2018