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A077678 Squarefree numbers beginning with 2. 12
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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

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

MATHEMATICA

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

PROG

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

CROSSREFS

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

Sequence in context: A106422 A320523 A171549 * A037309 A037315 A110301

Adjacent sequences:  A077675 A077676 A077677 * A077679 A077680 A077681

KEYWORD

base,easy,nonn

AUTHOR

Amarnath Murthy, Nov 16 2002

EXTENSIONS

More terms from Sascha Kurz, Jan 28 2003

STATUS

approved

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Last modified November 30 18:54 EST 2021. Contains 349424 sequences. (Running on oeis4.)