# A110808 (b-file synthesized from sequence entry) 1 1 2 2 3 6 4 24 5 120 6 6 7 5040 8 40320 9 362880 10 720 11 39916800 12 479001600 13 6227020800 14 87178291200 15 1307674368000 16 20922789888000 17 355687428096000 18 6402373705728000 19 121645100408832000 20 2432902008176640000