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 A080049 #22 Aug 31 2024 12:38:47 %S A080049 0,2,11,63,388,2734,21893,197069,1970726,21678036,260136487, %T A080049 3381774403,47344841720,710172625898,11362762014473,193166954246169, %U A080049 3477005176431178,66063098352192544,1321261967043851051,27746501307920872271,610423028774259190172,14039729661807961374198 %N A080049 Operation count to create all permutations of n distinct elements using Algorithm L (lexicographic permutation generation) from Knuth's The Art of Computer Programming, Vol. 4, chapter 7.2.1.2. Sequence gives number of interchange operations in step L4. %D A080049 Donald E. Knuth: The Art of Computer Programming, Volume 4, Fascicle 2, Generating All Tuples and Permutations. Addison-Wesley (2005). Chapter 7.2.1.2, 39-40. %H A080049 D. E. Knuth, <a href="https://www-cs-faculty.stanford.edu/~knuth/fasc2b.ps.gz">TAOCP Vol. 4, Pre-fascicle 2b (generating all permutations)</a>. %H A080049 R. J. Ord-Smith, <a href="https://doi.org/10.1093/comjnl/13.2.152">Generation of permutation sequences: Part 1</a>, Computer J., 13 (1970), 151-155. %H A080049 Hugo Pfoertner, <a href="https://www.randomwalk.de/sequences/lpure.txt">FORTRAN implementation of Knuth's Algorithm L for lexicographic permutation generation</a>. %F A080049 a(2)=0, a(n)=n*a(n-1) + (n-1)*floor((n-1)/2). %F A080049 c = limit n ->infinity a(n)/n! = 0.5430806.. = (e+1/e)/2-1 = A073743 - 1. %F A080049 a(n) = floor (c*n! - (n-1)/2) for n>=2. %o A080049 (Fortran) c FORTRAN program available at Pfoertner link. %Y A080049 Cf. A080047, A080048, A038155, A038156, A056542, A073743, A079756, A080047, A080048. %K A080049 nonn,easy %O A080049 2,2 %A A080049 _Hugo Pfoertner_, Jan 24 2003