A349424 Integers that are the product of two factorials A000142.

1, 2, 4, 6, 12, 24, 36, 48, 120, 144, 240, 576, 720, 1440, 2880, 4320, 5040, 10080, 14400, 17280, 30240, 40320, 80640, 86400, 120960, 241920, 362880, 518400, 604800, 725760, 967680, 2177280, 3628800, 4838400, 7257600, 8709120, 21772800, 25401600, 29030400

Offset

1,2

Comments

The first term a(1) = 0!*0! = 1!*0! = 1!*1!.

This is also the number of perfect matchings in a graph connecting C-G and A-U pairs in a RNA string (see Rosalind link). - Lorenzo Pieri, Mar 02 2023

Links

Table of n, a(n) for n=1..39.

Rosalind, Perfect Matchings and RNA Secondary Structures.

Example

720 is a term since 720 = 5!*3! = 6!*0! = 6!*1!.

Mathematica

(Union@Flatten@Table[x!*y!, {x, 0, 100}, {y, 0, x}])[[1 ;; 50]]

Prog

(PARI) lista(nn) = select(x->(x<2*nn!), setbinop((x, y)->x!*y!, [1..nn])); \\ Michel Marcus, Nov 17 2021

Crossrefs

Cf. A000142, A098361.

Subsequence of A001013.

Sequence in context: A309015 A355579 A357173 * A134865 A140753 A328519

Adjacent sequences: A349421 A349422 A349423 * A349425 A349426 A349427

Keyword

nonn

Author

Frank M Jackson, Nov 17 2021

Status

approved