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A341781 Refactorable numbers (or tau numbers, A033950) k such that k/tau(k) is even, where tau(k) = A000005(k). 2
8, 12, 36, 72, 80, 96, 128, 180, 240, 252, 288, 384, 396, 448, 468, 480, 560, 612, 640, 672, 684, 720, 828, 864, 880, 896, 972, 1040, 1044, 1056, 1116, 1152, 1200, 1248, 1332, 1344, 1360, 1408, 1440, 1476, 1520, 1548, 1620, 1632, 1664, 1680, 1692, 1800, 1824 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Zelinsky (2002) called these numbers p-generators. He proved that these are the tau numbers k such that for any prime p, if p does not divide k then p*k is also a tau number. He used these numbers to prove that the number of tau numbers not exceeding m is > pi(m)/2 for all m > 7.42*10^13, where pi(m) = A000720(m).

LINKS

Amiram Eldar, Table of n, a(n) for n = 1..10000

Joshua Zelinsky, Tau Numbers: A Partial Proof of a Conjecture and Other Results, Journal of Integer Sequences, Vol. 5 (2002), Article 02.2.8.

EXAMPLE

8 is a term since 8/tau(8) = 8/4 = 2 is even.

MATHEMATICA

q[n_] := Divisible[n, (d = DivisorSigma[0, n])] && EvenQ[n/d]; Select[Range[2000], q]

PROG

(PARI) isok(k) = my(q=k/numdiv(k)); (denominator(q)==1) && ((q%2) == 0); \\ Michel Marcus, Feb 20 2021

CROSSREFS

Cf. A000005, A000720, A033950, A036762, A039819, A281188.

Sequence in context: A045018 A067681 A132356 * A282943 A024604 A025103

Adjacent sequences: A341778 A341779 A341780 * A341782 A341783 A341784

KEYWORD

nonn

AUTHOR

Amiram Eldar, Feb 19 2021

STATUS

approved

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Last modified January 31 21:46 EST 2023. Contains 359981 sequences. (Running on oeis4.)