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A081520 Triangle read by rows in which row n consists of 1 and the n-1 smallest positive numbers having common factors with n.

%I A081520 #19 May 21 2015 14:41:05
%S A081520 1,1,2,1,3,6,1,2,4,6,1,5,10,15,20,1,2,3,4,6,8,1,7,14,21,28,35,42,1,2,
%T A081520 4,6,8,10,12,14,1,3,6,9,12,15,18,21,24,1,2,4,5,6,8,10,12,14,15,1,11,
%U A081520 22,33,44,55,66,77,88,99,110,1,2,3,4,6,8,9,10,12,14,15,16,1,13,26,39,52,65,78
%N A081520 Triangle read by rows in which row n consists of 1 and the n-1 smallest positive numbers having common factors with n.
%H A081520 Ivan Neretin, <a href="/A081520/b081520.txt">Table of n, a(n) for n = 1..5050</a>
%e A081520 1;
%e A081520 1, 2;
%e A081520 1, 3, 6;
%e A081520 1, 2, 4, 6;
%e A081520 1, 5,10,15,20;
%e A081520 1, 2, 3, 4, 6, 8;
%e A081520 1, 7,14,21,28,35,42;
%e A081520 ...
%p A081520 arow:= proc(n) local L;
%p A081520   L:= select(t -> igcd(t,n) > 1, [$1..n^2]);
%p A081520   1,op(L[1..n-1]);
%p A081520 end proc:
%p A081520 seq(arow(n),n=1..20); # _Robert Israel_, May 21 2015
%t A081520 Table[Join[{1}, Select[Range[n^2], GCD[n, #] > 1 &, n - 1]], {n, 1, 13}] // Flatten (* _Ivan Neretin_, May 21 2015 *)
%o A081520 (PARI) for(n=1,5,print1("\n1"); for(i=1,n-1,print1(", "i*n))) \\ _Charles R Greathouse IV_, May 21 2015
%Y A081520 Cf. A081518, A081519, A077661.
%K A081520 nonn,tabl,easy
%O A081520 1,3
%A A081520 _Amarnath Murthy_, Mar 27 2003
%E A081520 More terms from _Ryan Propper_, Nov 05 2005
%E A081520 Name edited by _Robert Israel_, May 21 2015