A124823 a(n) = n-th integer from among those positive integers which are coprime to n(n+1)/2.

1, 2, 7, 9, 8, 10, 15, 23, 16, 13, 35, 37, 16, 31, 53, 33, 26, 28, 49, 83, 40, 25, 71, 89, 33, 41, 95, 67, 56, 58, 63, 103, 58, 53, 151, 109, 40, 64, 157, 101, 73, 74, 95, 179, 88, 49, 143, 167, 71, 98, 175, 113, 80, 109, 173, 199, 92, 61, 223, 227, 64, 110, 221, 173, 142

Offset

1,2

Links

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

Example

The positive integers which are coprime to (5*6/2)=15 are 1,2,4,7,8,11,13,14... The fifth of these integers is 8, so a(5) = 8.

Maple

f:= proc(n) local t, k, count;
    t:= n*(n+1)/2; count:= 0;
    for k from 1 do
      if igcd(k, t)=1 then count:= count+1; if count = n then return k fi fi
    od
end proc:
map(f, [$1..100]); # Robert Israel, Mar 29 2018

Mathematica

f[n_] := Block[{k = 0, c = n}, While[c > 0, k++; While[GCD[n*(n + 1)/2, k] > 1, k++ ]; c--; ]; k]; Table[f[n], {n, 65}] (* Ray Chandler, Nov 10 2006 *)

Crossrefs

Cf. A124822.

Sequence in context: A363078 A114940 A120483 * A086725 A019950 A201892

Adjacent sequences: A124820 A124821 A124822 * A124824 A124825 A124826

Keyword

nonn,look

Author

Leroy Quet, Nov 08 2006

Extensions

Extended by Ray Chandler, Nov 10 2006

Status

approved