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A080203 Steffi sequence; the numbers of pairs of unequal permutations of all the digits 1, ..., b-1 in base b whose ratio is an integer. 1
0, 0, 1, 1, 25, 7, 623, 1183, 24603, 5895, 2017603, 242283 (list; graph; refs; listen; history; text; internal format)
OFFSET
2,5
COMMENTS
Terms computed by Michael Trott.
LINKS
Eric Weisstein's World of Mathematics, Steffi Problem
EXAMPLE
In base 2, there are no solutions for 1, so a(2)=0. In base 3, there are no solutions for 12, so a(3)=0. In base 4, there is a single solutions for 123, so a(4)=1. In base 5, there is a single solution with the digits 1234: 4312_5/1234_5 = 3, so a(5)=1.
MATHEMATICA
f[n_] := Block[{p = FromDigits[ #, n] & /@ Permutations[ Range[n - 1]]}, Sum[ Length[ Select[ p/p[[i]], IntegerQ[ # ] &]], {i, (n - 1)!}] - (n - 1)! ]; Table[ f[n], {n, 2, 9}] (* Robert G. Wilson v, Jul 15 2005 *)
CROSSREFS
T(b-1, b) entries of A080202.
Sequence in context: A243092 A126837 A248583 * A040605 A307179 A065910
KEYWORD
nonn,hard,base,more
AUTHOR
Eric W. Weisstein, Feb 05 2003
EXTENSIONS
Prepended a(2)=0, added a(13) - Giovanni Resta, Feb 24 2013
STATUS
approved

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Last modified June 15 00:47 EDT 2024. Contains 373402 sequences. (Running on oeis4.)