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

1,3

Links

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

Example

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;
...

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 *)

Prog

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

Crossrefs

Cf. A081518, A081519, A077661.

Sequence in context: A077890 A077877 A144867 * A238955 A238968 A217891

Adjacent sequences: A081517 A081518 A081519 * A081521 A081522 A081523

Keyword

nonn,tabl,easy

Author

Amarnath Murthy, Mar 27 2003

Extensions

More terms from Ryan Propper, Nov 05 2005

Name edited by Robert Israel, May 21 2015

Status

approved