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A262443 Positive integers m such that pi(m^2) = pi(j^2)*pi(k^2) for some 0 < j < k < m, where pi(x) denotes the number of primes not exceeding x. 9
8, 11, 14, 19, 20, 36, 38, 45, 66, 87, 91, 115, 139, 143, 152, 155, 201, 220, 227, 279, 357, 383, 391, 415, 418, 452, 476, 480, 489, 496, 500, 514, 521, 524, 549, 552, 557, 588, 595, 632, 653, 676, 706, 708, 749, 753, 761, 766, 820, 846, 863, 877, 922, 1009, 1038, 1041, 1044, 1052, 1057, 1080 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Conjecture: (i) The sequence has infinitely many terms. Also, there are infinitely many positive integers m such that pi(m^2) = pi(j^2)*pi(k^2) for no 0 < j <= k < m.

(ii) For any integer n > 2, the equation pi(x^n)*pi(y^n) = pi(z^n) has no solution with 0 < x <= y < z.

REFERENCES

Zhi-Wei Sun, Problems on combinatorial properties of primes, in: M. Kaneko, S. Kanemitsu and J. Liu (eds.), Number Theory: Plowing and Starring through High Wave Forms, Proc. 7th China-Japan Seminar (Fukuoka, Oct. 28 - Nov. 1, 2013), Ser. Number Theory Appl., Vol. 11, World Sci., Singapore, 2015, pp. 169-187.

LINKS

Zhi-Wei Sun, Table of n, a(n) for n = 1..300

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

EXAMPLE

a(1) = 8 since pi(8^2) = pi(64) = 18 = 2*9 = pi(2^2)*pi(5^2) with 0 < 2 < 5 < 8.

a(4) = 19 since pi(19^2) = pi(361) = 72 = 4*18 = pi(3^2)*pi(8^2) with 0 < 3 < 8 < 19.

MATHEMATICA

f[n_]:=PrimePi[n^2]

T[n_]:=Table[f[k], {k, 1, n}]

Dv[n_]:=Divisors[f[n]]

Le[n_]:=Length[Dv[n]]

n=0; Do[Do[If[MemberQ[T[m], Part[Dv[m], i]]&&MemberQ[T[m], Part[Dv[m], Le[m]-i+1]], n=n+1; Print[n, " ", m]; Goto[aa]], {i, 2, (Le[m]-1)/2}]; Label[aa]; Continue, {m, 1, 1080}]

CROSSREFS

Cf. A000290, A000720, A038107, A262408, A262409.

Sequence in context: A190208 A061570 A096679 * A287552 A275191 A279776

Adjacent sequences:  A262440 A262441 A262442 * A262444 A262445 A262446

KEYWORD

nonn

AUTHOR

Zhi-Wei Sun, Sep 23 2015

STATUS

approved

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Last modified December 14 01:15 EST 2019. Contains 329977 sequences. (Running on oeis4.)