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A121052 Smallest positive integer m for which n^m is congruent to 1 modulo n^2+n-1. 2
1, 4, 5, 9, 14, 40, 20, 35, 44, 108, 65, 60, 45, 90, 119, 135, 60, 30, 189, 209, 46, 100, 63, 299, 145, 700, 100, 135, 390, 928, 99, 84, 522, 280, 629, 605, 56, 1480, 779, 740, 430, 684, 60, 989, 517, 80, 40, 1175, 195, 2548, 240, 252, 715, 424, 81, 1595, 220, 310 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

The sequence arises as the order of a shuffle of n(n+1) cards in which cards are laid out in an array of n+1 rows of n columns; cards are picked up by column and laid out by rows.

More generally there is a function of two variables, f(r,c) for which f(r,c) is the least integer such that c^f(r,c) is congruent to 1 modulo rc-1. Of interest is the ratio of phi(rc-1)/f(r,c) or in the case of the sequence proposed, phi(n^2+n-1)/m.

I would like to know if there is some direct way to predict these orders, or the ratio of phi(rc-1)/f(r,c). The program provided produces the table f(r,c).

LINKS

Table of n, a(n) for n=1..58.

EXAMPLE

a(2)=4 because 2^4=16=1 mod 5 but 2^1, 2^2 and 2^3 are not;

a(3)=5 because 3^5=1 mod 11 and 5 is the smallest such.

MAPLE

TAB:=proc(Rmin, Rmax, Cmin, Cmax) local r, c, T, m, ct, A; T:=array(1..Rmax-Rmin+1, 1..Cmax-Cmin+1); for r from Rmin to Rmax do for c from Cmin to Cmax do A:=c; ct:=1; m:=r*c-1; while not A = 1 do A:=A*c mod m; ct:=ct+1; od; T[r-Rmin+1, c-Cmin+1]:=[ct, phi(m)]; od; od; eval(T) end:

MATHEMATICA

f[n_] := If[n == 1, 1, Block[{m = 1, k = n^2 + n - 1}, While[Mod[n^m, k] != 1, m++ ]; m]]; Array[f, 59] (* Robert G. Wilson v *)

PROG

(PARI) print1(1, ", "); for(n=2, 60, q=n^2+n-1; m=1; while(lift(Mod(n, q)^m)!=1, m++); print1(m, ", ")) - (Klaus Brockhaus, Aug 09 2006)

CROSSREFS

Sequence in context: A250474 A042765 A041353 * A041823 A042489 A217685

Adjacent sequences:  A121049 A121050 A121051 * A121053 A121054 A121055

KEYWORD

nonn

AUTHOR

John H. Mason, Aug 09 2006

EXTENSIONS

More terms from Klaus Brockhaus and Robert G. Wilson v, Aug 09 2006

STATUS

approved

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Last modified October 22 18:02 EDT 2019. Contains 328319 sequences. (Running on oeis4.)