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 A247394 Numbers n for which the third maximal prime <= sqrt(n) is the least prime divisor of n. 10
 26, 28, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48, 51, 57, 63, 69, 75, 81, 87, 93, 99, 105, 111, 117, 125, 145, 155, 203, 217, 259, 287, 319, 341, 377, 403, 481, 629, 697, 731, 799, 893, 989, 1081, 1219, 1357, 1537, 1829, 1961, 2183, 2419, 2501, 2747, 2881, 3053 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS These numbers we call "preprimes" of the third kind in contrast to A156759 for n>=2, for which the maximal prime <= sqrt(n) is the least prime divisor of n; and to A247393 for which the second maximal prime <= sqrt(n) is the least prime divisor of n. LINKS Jens Kruse Andersen, Table of n, a(n) for n = 1..10000 Vladimir Shevelev, A classification of the positive integers over primes FORMULA lpf(a(n)) = prime(pi(isqrt(a(n))-2), with pi(n) = A000720(n), lpf(n) = A020639(n) and isqrt(n) = A000196(n). MATHEMATICA Select[Range, Prime[PrimePi[Sqrt[#]]-2] == FactorInteger[#][[1, 1]] &] (* Indranil Ghosh, Mar 08 2017 *) PROG (PARI) select(n->prime(primepi(sqrtint(n))-2)==factor(n)[1, 1], vector(10^4, x, x+24)) \\ Jens Kruse Andersen, Sep 17 2014 CROSSREFS Cf. A156759, A247393. Sequence in context: A042400 A042401 A042402 * A056081 A081644 A256619 Adjacent sequences:  A247391 A247392 A247393 * A247395 A247396 A247397 KEYWORD nonn AUTHOR Vladimir Shevelev, Sep 16 2014 EXTENSIONS Terms up to a(54) from Peter J. C. Moses, Sep 16 2014 STATUS approved

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Last modified July 13 03:03 EDT 2020. Contains 335673 sequences. (Running on oeis4.)