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A238645 Number of odd primes p < 2*n such that the number of squarefree numbers among 1, ..., ((p-1)/2)*n is prime. 3
0, 1, 2, 1, 2, 2, 1, 2, 2, 5, 2, 3, 3, 1, 6, 5, 3, 3, 1, 4, 2, 4, 4, 3, 4, 2, 4, 3, 1, 4, 3, 3, 7, 5, 4, 5, 5, 4, 3, 2, 5, 2, 2, 4, 5, 4, 9, 7, 4, 3, 2, 4, 3, 4, 3, 2, 4, 6, 5, 6, 4, 4, 2, 2, 7, 5, 6, 6, 8, 3, 7, 3, 5, 6, 10, 6, 6, 6, 4, 5 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

Conjecture: a(n) > 0 for all n > 1, and a(n) = 1 only for n = 2, 4, 7, 14, 19, 29.

This is an analog of the conjecture in A237578 for squarefree numbers. We have verified it for n up to 20000.

See also A238646 for a similar conjecture involving squarefree numbers.

LINKS

Zhi-Wei Sun, Table of n, a(n) for n = 1..900

Zhi-Wei Sun, Problems on combinatorial properties of primes, arXiv:1402.6641, 2014.

EXAMPLE

a(4) = 1 since 3 is prime and there are exactly 3 squarefree numbers among 1, ..., (3-1)/2*4 (namely, 1, 2, 3).

a(14) = 1 since 5 and 17 are both prime, and there are exactly 17 squarefree numbers among 1, ..., (5-1)/2*14.

a(19) = 1 since 3 and 13 are both prime, and there are exactly 13 squarefree numbers among 1, ..., (3-1)/2*19 (namely, 1, 2, 3, 5, 6, 7, 10, 11, 13, 14, 15, 17, 19).

a(29) = 1 since 41 and 353 are both prime, and there are exactly 353 squarefree numbers among 1, ..., (41-1)/2*29 = 580.

MATHEMATICA

s[n_]:=Sum[If[SquareFreeQ[k], 1, 0], {k, 1, n}]

a[n_]:=Sum[If[PrimeQ[s[(Prime[k]-1)/2*n]], 1, 0], {k, 2, PrimePi[2n-1]}]

Table[a[n], {n, 1, 80}]

CROSSREFS

Cf. A000040, A005117, A013928, A237578, A237615, A238646.

Sequence in context: A278043 A014643 A236265 * A118382 A007723 A067437

Adjacent sequences:  A238642 A238643 A238644 * A238646 A238647 A238648

KEYWORD

nonn

AUTHOR

Zhi-Wei Sun, Mar 02 2014

STATUS

approved

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Last modified July 12 00:27 EDT 2020. Contains 335658 sequences. (Running on oeis4.)