A077678 Squarefree numbers beginning with 2.

2, 21, 22, 23, 26, 29, 201, 202, 203, 205, 206, 209, 210, 211, 213, 214, 215, 217, 218, 219, 221, 222, 223, 226, 227, 229, 230, 231, 233, 235, 237, 238, 239, 241, 246, 247, 249, 251, 253, 254, 255, 257, 258, 259, 262, 263, 265, 266, 267, 269, 271, 273, 274

Offset: 1

Author

Amarnath Murthy, Nov 16 2002

Keywords

• base
• easy
• nonn

Comments

Intersection of A005117 and A217394. - Michel Marcus, Sep 14 2013

Lower density is 3/(10*Pi^2), upper density is 20/(9*Pi^2). - Charles R Greathouse IV, Nov 05 2017

Links

• Amiram Eldar, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A077677, A077679, A077680, A077681, A077682, A077683, A077684, A077685.

Programs

Mathematica

s2Q[n_]:=First[IntegerDigits[n]]==2 && 1==Max@@Last/@FactorInteger[n]; Select[Range[274],s2Q[#] &] (* Jayanta Basu, May 22 2013 *)

PARI

is(n)=n>1 && digits(n)[1]==2 && issquarefree(n) \\ Charles R Greathouse IV, Nov 05 2017

Extensions

More terms from Sascha Kurz, Jan 28 2003