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A124823
a(n) = n-th integer from among those positive integers which are coprime to n(n+1)/2.
3
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
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: A377479 A114940 A120483 * A086725 A019950 A201892
KEYWORD
nonn,look
AUTHOR
Leroy Quet, Nov 08 2006
EXTENSIONS
Extended by Ray Chandler, Nov 10 2006
STATUS
approved