A081520 - OEIS

%I #19 May 21 2015 14:41:05

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

%H Ivan Neretin, <a href="/A081520/b081520.txt">Table of n, a(n) for n = 1..5050</a>

%e 1;

%e 1, 2;

%e 1, 3, 6;

%e 1, 2, 4, 6;

%e 1, 5,10,15,20;

%e 1, 2, 3, 4, 6, 8;

%e 1, 7,14,21,28,35,42;

%e ...

%p arow:= proc(n) local L;

%p   L:= select(t -> igcd(t,n) > 1, [$1..n^2]);

%p   1,op(L[1..n-1]);

%p end proc:

%p seq(arow(n),n=1..20); # _Robert Israel_, May 21 2015

%t Table[Join[{1}, Select[Range[n^2], GCD[n, #] > 1 &, n - 1]], {n, 1, 13}] // Flatten (* _Ivan Neretin_, May 21 2015 *)

%o (PARI) for(n=1,5,print1("\n1"); for(i=1,n-1,print1(", "i*n))) \\ _Charles R Greathouse IV_, May 21 2015

%Y Cf. A081518, A081519, A077661.

%K nonn,tabl,easy

%O 1,3

%A _Amarnath Murthy_, Mar 27 2003

%E More terms from _Ryan Propper_, Nov 05 2005

%E Name edited by _Robert Israel_, May 21 2015