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A161424 Numbers k whose largest divisor <= sqrt(k) equals 4. 27
16, 20, 24, 28, 32, 44, 52, 68, 76, 92, 116, 124, 148, 164, 172, 188, 212, 236, 244, 268, 284, 292, 316, 332, 356, 388, 404, 412, 428, 436, 452, 508, 524, 548, 556, 596, 604, 628, 652, 668, 692, 716, 724, 764, 772, 788, 796, 844, 892, 908, 916, 932, 956, 964 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Define a sieve operation with parameter s that eliminates integers of the form s^2 + s*i (i >= 0) from the set A000027 of natural numbers. The sequence lists those natural numbers that are eliminated by the sieve s=4 and cannot be eliminated by any sieve s >= 5. - R. J. Mathar, Jun 24 2009

See A161344 for more information. - Omar E. Pol, Jul 05 2009

See also the array in A163280, the main entry for this sequence. - Omar E. Pol, Oct 24 2009

LINKS

Table of n, a(n) for n=1..54.

Omar E. Pol, Determinacion geometrica de los numeros primos y perfectos

Omar E. Pol, Illustration: Divisors and pi(x)

Omar E. Pol, Illustration for A008578, A161344, A161345 and A161424 [From Omar E. Pol, Oct 24 2009]

FORMULA

Numbers n such that A033676(n)=4. - Omar E. Pol, Jul 05 2009

MAPLE

isA := proc(n, s) if n mod s <> 0 then RETURN(false); fi; if n/s-s >= 0 then RETURN(true); else RETURN(false); fi; end: isA161424 := proc(n) for s from 5 to n do if isA(n, s) then RETURN(false); fi; od: isA(n, 4) ; end: for n from 1 to 3000 do if isA161424(n) then printf("%d, ", n) ; fi; od; # R. J. Mathar, Jun 24 2009

MATHEMATICA

Select[Range[1, 1000], Function[m, Max[Select[Divisors[m], # <= Sqrt[m] &]] == 4]] (* Ashton Baker, Nov 03 2013 *)

CROSSREFS

Cf. A000005, A018253, A160811, A160812, A161205, A161344, A161345, A161346, A161425, A161428, A033676, A008578, A161835, A162526, A162527, A162528, A162529, A162530, A162531, A162532.

Cf. Fourth column of array in A163280. Also, fourth row of array in A163990. - Omar E. Pol, Oct 24 2009

Sequence in context: A163901 A044860 A272533 * A248428 A088449 A104010

Adjacent sequences:  A161421 A161422 A161423 * A161425 A161426 A161427

KEYWORD

easy,nonn

AUTHOR

Omar E. Pol, Jun 20 2009

EXTENSIONS

Terms beyond a(8) from R. J. Mathar, Jun 24 2009

Definition added by R. J. Mathar, Jun 28 2009

STATUS

approved

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Last modified February 16 16:00 EST 2020. Contains 331961 sequences. (Running on oeis4.)