A333004 - OEIS

A333004

a(n) is the smallest squarefree number k such that sum of reciprocals of squarefree numbers up to k (squarefree harmonic sum) exceeds n.

1, 2, 5, 26, 130, 670, 3466, 17985, 93179, 482762, 2501013, 12956855, 67125243, 347753857, 1801596939

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OFFSET

0,2

COMMENTS

The corresponding indices of a(n) in A005117 are 1, 2, 4, 17, 80, 409, 2112, 10936, 56651, 293483, 1520429, ...

LINKS

Eric Weisstein's World of Mathematics, Squarefree

EXAMPLE

a(2) = 5 because 1/1 + 1/2 + 1/3 = 1.8333... < 2 but 1/1 + 1/2 + 1/3 + 1/5 = 2.0333... > 2.

CROSSREFS

Similar sequences: A002387 (for positive integers), A016088 (for primes), A076751 (for composite numbers), A332991 (for prime powers).

Cf. A005117 (squarefree numbers).

KEYWORD

nonn,hard,more

AUTHOR

EXTENSIONS

a(11)-a(14) from Daniel Suteu, Mar 05 2020

STATUS

approved