A336340 - OEIS

%I #8 Oct 03 2020 15:33:54

%S 1,3,5,7,8,9,10,11,12,14,16,17,18,21,24,25,26,27,28,30,31,32,36,37,38,

%T 39,40,41,48,49,52,57,60,61,62,64,65,66,69,74,76,82,83,84,85,86,87,89,

%U 91,92,93,94,95,96,97,98,101,102,103,106,109,110,111,112

%N a(n) = (1/2)A336338(n).

%e In the following table, c(n) = A002808(n) = composite(n).

%e   n     c(n)   gcd(n, c(n))

%e   1      4         1

%e   2      6         2

%e   3      8         1

%e   4      9         1

%e   5      10        5

%e   6      12        6

%e 2 and 6 are in A336338; 6 and 12 are in A336339; 1 and 3 are in A336340; 3 and 6 are in A336341.

%t c = Select[Range[2, 200], ! PrimeQ[#] &];  (*  A002808 *)

%t u = Select[Range[Length[c]], EvenQ[GCD[c[[#]], #]] &]   (* A336338 *)

%t v = Table[c[[u[[n]]]], {n, 1, Length[u]}]; (* A336339 *)

%t u/2  (* A336340 *)

%t v/2  (* A336341 *)

%Y Cf. A002808, A336338, A336339, A336341.

%K nonn,easy

%O 1,2

%A _Clark Kimberling_, Oct 03 2020