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

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, 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, 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

Offset: 1

Author

Amarnath Murthy, Mar 27 2003

Keywords

• nonn
• tabl
• easy

Examples

			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;
...
		

Links

• Ivan Neretin, Table of n, a(n) for n = 1..5050

Crossrefs

Cf. A081518, A081519, A077661.

Programs

Maple

arow:= proc(n) local L;
  L:= select(t -> igcd(t,n) > 1, [$1..n^2]);
  1,op(L[1..n-1]);
end proc:
seq(arow(n),n=1..20); # Robert Israel, May 21 2015

Mathematica

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

PARI

for(n=1,5,print1("\n1"); for(i=1,n-1,print1(", "i*n))) \\ Charles R Greathouse IV, May 21 2015

Extensions

More terms from Ryan Propper, Nov 05 2005

Name edited by Robert Israel, May 21 2015