A065804 Earliest sequence with a(a(n))=n!.
1, 1, 2, 4, 6, 7, 24, 120, 9, 40320, 11, 3628800, 13, 479001600, 15, 87178291200, 17, 20922789888000, 19, 6402373705728000, 21, 2432902008176640000, 23, 1124000727777607680000, 720, 26, 15511210043330985984000000, 28
Offset: 0
Keywords
Examples
a(3)=4 since any lower value would suggest a(a(3)) was not 3!=6. So a(4)=a(a(3))=3!=6. a(5)=7 since any lower value would prevent a(a(5))=5!. a(6)=a(3!)=a(a(a(3)))=a(a(4))=4!=24. a(7)=a(a(5))=5!=120. a(8)=9 since any lower value would prevent a(a(8))=8!. etc.