A155891 - OEIS

A155891

Least positive integer such that a(n)! starts with n, both written in binary.

1, 2, 3, 7, 6, 3, 5, 20, 7, 14, 6, 4, 10, 12, 5, 20, 27, 50, 7, 14, 23, 6, 13, 4, 25, 30, 10, 12, 51, 5, 63, 24, 20, 36, 27, 50, 59, 11, 7, 14, 26, 75, 23, 9, 6, 13, 49, 71, 144, 31, 25, 30, 199, 19, 10, 46, 12, 51, 83, 5, 28, 73, 63, 62, 24, 54, 20, 36, 67, 27, 40, 74, 50, 59, 124

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,2

LINKS

Nick Hobson, Table of n, a(n) for n = 1..16383

Nick Hobson, C program.

EXAMPLE

a(7) = 5 since 7 = 111_2 (i.e., written in base 2) and 5! = 1111000_2 is the least factorial to have 111 as its most significant binary digits.

PROG

(PARI) A155891(n)={ local( F=1, k=1 ); while( F\1!=n, F*=k++; while( F>=n+1, F/=2 )); k}

CROSSREFS

Cf. A018799, A076219.