A131880 - OEIS

A131880

a(n) = the n-th positive integer which is coprime to (2^n -1). a(n) = the n-th term of row n in triangle A131879.

1, 2, 3, 7, 5, 10, 7, 14, 10, 16, 11, 31, 13, 20, 17, 31, 17, 32, 19, 46, 24, 34, 23, 62, 25, 38, 31, 56, 29, 61, 31, 62, 39, 50, 36, 97, 37, 56, 45, 97, 41, 74, 43, 86, 53, 70, 47, 122, 49, 85, 59, 98, 53, 103, 58, 122, 66, 88, 59, 173, 61, 92, 74, 127, 67, 125, 67, 127, 81

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OFFSET

1,2

COMMENTS

If n is prime then a(n) = n. - Robert Israel, Dec 19 2017

LINKS

Robert Israel, Table of n, a(n) for n = 1..10000

EXAMPLE

2^6 -1 = 63. a(6) is the 6th positive integer which is coprime to 63. So a(6) = 10.

MAPLE

f:= proc(n) local t, k, count;

if isprime(n) then return n fi;

t:= 2^n-1; count:= 0;

for k from 1 do

if igcd(k, t) = 1 then

count:= count+1;

if count = n then return k fi

end proc:

map(f, [$1..100]); # Robert Israel, Dec 19 2017

MATHEMATICA

a[n_] := Module[{cnt = 0, k}, For[k = 1, True, k++, If[CoprimeQ[2^n-1, k], cnt++; If[cnt == n, Return[k]]]]];

Table[a[n], {n, 1, 100}] (* Jean-François Alcover, Feb 07 2023 *)

CROSSREFS

Cf. A000225, A131879.

Sequence in context: A223702 A120726 A060203 * A045790 A159842 A085102

Adjacent sequences: A131877 A131878 A131879 * A131881 A131882 A131883

KEYWORD

nonn

AUTHOR

Leroy Quet, Jul 23 2007

EXTENSIONS

More terms from R. J. Mathar, Aug 01 2007

STATUS

approved