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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
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.
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[4000], 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
Sequence in context: A042400 A042401 A042402 * A056081 A081644 A256619
KEYWORD
nonn
AUTHOR
Vladimir Shevelev, Sep 16 2014
EXTENSIONS
Terms up to a(54) from Peter J. C. Moses, Sep 16 2014
STATUS
approved