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A121998 Table, n-th row gives numbers between 1 and n that have a common factor with n.

%I A121998 #9 Mar 13 2018 22:13:18
%S A121998 2,3,2,4,5,2,3,4,6,7,2,4,6,8,3,6,9,2,4,5,6,8,10,11,2,3,4,6,8,9,10,12,
%T A121998 13,2,4,6,7,8,10,12,14,3,5,6,9,10,12,15,2,4,6,8,10,12,14,16,17,2,3,4,
%U A121998 6,8,9,10,12,14,15,16,18,19,2,4,5,6,8,10,12,14,15,16,18,20,3,6,7,9,12,14,15
%N A121998 Table, n-th row gives numbers between 1 and n that have a common factor with n.
%C A121998 Row n contains numbers m <= n such that gcd(m,n) > 1, i.e., numbers m in the cototient of n. - _Michael De Vlieger_, Mar 13 2018
%H A121998 Michael De Vlieger, <a href="/A121998/b121998.txt">Table of n, a(n) for n = 2..12949</a> (rows 2 <= n <= 256).
%e A121998 2;
%e A121998 3;
%e A121998 2,4;
%e A121998 5;
%e A121998 2,3,4,6;
%e A121998 7;
%e A121998 ...
%t A121998 Table[Select[Range@ n, ! CoprimeQ[#, n] &], {n, 20}] // Flatten (* _Michael De Vlieger_, Mar 13 2018 *)
%Y A121998 Cf. A051953 (row lengths), A038566, A081520, A133995 (nondivisors in the cototient of n).
%K A121998 nonn,tabf
%O A121998 2,1
%A A121998 _Franklin T. Adams-Watters_, Sep 11 2006