

A104564


Number of distinct prime divisors of 77...771 (with n 7's).


6



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, 4, 3, 4, 4, 6, 2, 6, 5, 4, 4, 4, 5, 3, 3, 5, 2, 6, 3, 5, 5, 4, 6, 4, 6, 5, 3, 3, 4, 4, 4, 4, 6, 4, 3, 6, 5, 3
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OFFSET

1,2


COMMENTS

Also number of distinct prime factors of (10^(n + 1)  1)*7/9  6.  Stefan Steinerberger, Mar 01 2006


LINKS

Amiram Eldar, Table of n, a(n) for n = 1..231


FORMULA

a(n) = A001221(A173806(n+1)).  Amiram Eldar, Jan 24 2020


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. A001221, A104484, A104483, A173806.
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
Offset corrected and more terms added by Amiram Eldar, Jan 24 2020


STATUS

approved



