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A104564 Number of distinct prime divisors of 77...771 (with n 7s). 5
1, 2, 2, 2, 3, 2, 2, 2, 2, 3, 3, 1, 3, 4, 2, 2, 3, 2, 1, 4, 3, 1, 3, 3, 2, 3, 4, 2, 7, 1, 5, 6, 3, 3, 4, 3, 4, 5, 5, 3, 5, 4, 2, 3, 3, 6, 2, 2, 6, 5 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

COMMENTS

Also numbers n such that 10^(n + 1) - 1)*7/9 - 6 is prime. - Stefan Steinerberger, Mar 01 2006

LINKS

Table of n, a(n) for n=0..49.

EXAMPLE

The number of distinct prime divisors of 71 is 1 (prime).

The number of distinct prime divisors of 771 is 2.

The number of distinct prime divisors of 7771 is 2.

MATHEMATICA

Table[Length[FactorInteger[(10^(n + 1) - 1)*7/9 - 6]], {n, 1, 50}] (* Stefan Steinerberger, Mar 01 2006 *)

PrimeNu/@(FromDigits/@Table[PadLeft[{1}, n, 7], {n, 2, 55}])  (* Harvey P. Dale, Apr 22 2011 *)

CROSSREFS

Cf. A104484, A104483.

Cf. A104484 [3 instead of 7], A104659 [4 instead of 7], A104517 [5 instead of 7], A104890 [6 instead of 7], A105972 [8 instead of 7], A105259 [9 instead of 7].

Sequence in context: A089367 A130192 A175064 * A160764 A156384 A306249

Adjacent sequences:  A104561 A104562 A104563 * A104565 A104566 A104567

KEYWORD

nonn,base

AUTHOR

Parthasarathy Nambi, Apr 20 2005

EXTENSIONS

More terms from Stefan Steinerberger, Mar 01 2006

STATUS

approved

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Last modified May 24 09:41 EDT 2019. Contains 323529 sequences. (Running on oeis4.)