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A089361 Numbers of pairs (i, j), i, j > 1, such that i^j <= n. 3
0, 0, 0, 1, 1, 1, 1, 2, 3, 3, 3, 3, 3, 3, 3, 5, 5, 5, 5, 5, 5, 5, 5, 5, 6, 6, 7, 7, 7, 7, 7, 8, 8, 8, 8, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 15, 15, 15, 15, 15, 15 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,8

COMMENTS

These numbers are related to the divergent series r sum(n^(1/k)) = n^(1/2) + n^(1/3) + ... + n^(1/r) for abs(n) > 0 and r=sqrt(n). Notice some numbers are missing, such as 4, 11, 12, 14.

Gaps (i.e., a(n) - a(n-1) > 1) occur for values of n > 1 in A117453. a(n) - a(n-1) = number of factors of j > 1, for the j in the pair (i,j) with the smallest value of i. Where n = A117453(x), a(n) = a(n-1) + A175066(x). For example: n = 64, a(64) = 13, a(63) = 10, 13 - 10 = 3; 64 = 2^6, 6 has three factors (2,3,6), corresponding to the three perfect powers for 64 (2^6, 4^3, 8^2). Also, A117453(3) = 64 and A175066(3) = 3. - Doug Bell, Jun 23 2015

LINKS

Robert Israel, Table of n, a(n) for n = 1..10000

FORMULA

a(1) = 0; for n > 1, if n not in A001597, a(n) = a(n-1), otherwise a(n) = a(n-1) + number of factors of j > 1 (A000005(j) - 1), for the j in the positive integer pair (i,j) where i^j = n with the smallest value of i. - Doug Bell, Jun 23 2015

a(n) = Sum(j = 2 .. floor(log[2](n)), floor(n^(1/j) - 1)). - Robert Israel, Jun 24 2015

EXAMPLE

There are 5 perfect powers greater than 1 that are less than or equal 16: 2^2, 2^3, 2^4, 3^2, 4^2, ergo the first 5 in the table.

MAPLE

N:= 1000; # to get a(1) to a(N)

B:= Vector(N);

for i from 2 to floor(sqrt(N)) do

  for j from 2 while i^j <= N do

    B[i^j]:= B[i^j]+1

  od

od:

convert(map(round, Statistics:-CumulativeSum(B)), list); # Robert Israel, Jun 24 2015

PROG

(PARI) plessn(n, m) = { for(k=1, n, s=0; r = floor(sqrt(k)); for(x=m, r, for(y=2, r, p = floor(x^y); if(p<=k, s++) ) ); print1(s", ") ) }

(PARI) A = vector(100); for (p = 2, 6, i = 2; while (i^p <= 100, A[i^p]++; i++)); for (n = 2, 100, A[n] += A[n - 1]); \\ David Wasserman

CROSSREFS

Cf. A001597, A117453, A175066. - Doug Bell, Jun 23 2015

Sequence in context: A025789 A080585 A295977 * A035441 A025784 A035390

Adjacent sequences:  A089358 A089359 A089360 * A089362 A089363 A089364

KEYWORD

easy,nonn

AUTHOR

Cino Hilliard, Dec 27 2003

STATUS

approved

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Last modified February 25 18:40 EST 2020. Contains 332256 sequences. (Running on oeis4.)