OFFSET
1,3
EXAMPLE
The positive integers coprime to a(k) and <= a(k), for 1<=k<=8, are for a(1):{1}, for a(2):{1}, for a(3):{1}, for a(4):{1,2}, for a(5):{1,3}, for a(6):{1,2}, for a(7):{1,5} and for a(8):{1,2,3,4}.
Those terms a(k), 1<=k<=8, which don't have any integers which are not coprime to 9 among those positive integers which are <=a(k) and coprime to a(k) are the six terms a(1)=1,a(2)=1,a(3)=2,a(4)=3,a(6)=3 and a(7)=6. So a(9) = 6.
MATHEMATICA
f[n_] := Select[Range[n], GCD[ #, n] == 1 &]; g[l_List] := Block[{fn = f[Length[l] + 1]}, Append[l, Length@Select[l, Union[f[ # ], fn] == fn &]]]; Nest[g, {1}, 80] (* Ray Chandler, Dec 21 2006 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Leroy Quet, Dec 20 2006
EXTENSIONS
Extended by Ray Chandler, Dec 21 2006
STATUS
approved