A126287 a(1) = 1, a(2) = 1, a(n) = n * LeastPrimeFactor(n-1) / LeastPrimeFactor(n).

1, 1, 2, 6, 2, 15, 2, 28, 6, 15, 2, 66, 2, 91, 10, 24, 2, 153, 2, 190, 14, 33, 2, 276, 10, 65, 18, 42, 2, 435, 2, 496, 22, 51, 14, 90, 2, 703, 26, 60, 2, 861, 2, 946, 30, 69, 2, 1128, 14, 175, 34, 78, 2, 1431, 22, 140, 38, 87, 2, 1770, 2, 1891, 42, 96, 26, 165, 2, 2278, 46, 105

Offset: 1

Author

Lior Manor, Dec 25 2006

Keywords

• easy
• nonn

Examples

			a(6) = 6 * LeastPrimeFactor(5) / LeastPrimeFactor(6) = 6 * 5 / 2 = 15
		

Links

• Ivan Neretin, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A126286, A126288, A126289, A020639.

Programs

Maple

N:= 1000: # to get a(1) to a(n)
a[1]:= 1: a[2]:= 1:
b:= 2:
for n from 3 to N do
c:= min(numtheory:-factorset(n));
a[n]:= n*b/c;
b:= c;
od:
seq(a[n],n=1..N); # Robert Israel, May 20 2015

Mathematica

a[1] := 1; a[2] := 1; a[n_] := n*FactorInteger[n - 1][[1, 1]]/FactorInteger[n][[1, 1]]; Table[a[n], {n, 70}] (* Ivan Neretin, May 20 2015 *)

Formula

a(n) = A032742(n)*A020639(n-1), for n>2. - Michel Marcus, May 20 2015