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A245787 Numbers n such that tau(n)*sigma(n) divides n^2. 1
1, 26208, 56896, 293760, 997920, 9694080, 23569920, 25159680, 29669760, 67858560, 117849600, 132723360, 208565280, 222963840, 276756480, 427714560, 513786240, 578672640, 628992000, 649503360, 688279680, 714954240, 779950080, 830269440, 979102080, 1037266560 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,2
COMMENTS
a(11) > 10^8. - Derek Orr, Aug 08 2014
Numbers n such that n^2 / (A000005(n) * A000203(n)) is integer.
Subsequence of A090777 (numbers n such that sigma(n) divides n^2).
Sequence of numbers k(n) = n^2 / (tau(n) * sigma(n)): 1, 104, 889, 612, 945, 7344, …
LINKS
Jens Kruse Andersen, Table of n, a(n) for n = 1..47
MATHEMATICA
a245787[n_Integer] := Select[Range[n], Divisible[#^2, DivisorSigma[0, #]*DivisorSigma[1, #]]&] (* Michael De Vlieger, Aug 09 2014 *)
PROG
(Magma) [n: n in [1..1000000] | Denominator(n^2 / ((#[d: d in Divisors(n)])* SumOfDivisors(n))) eq 1]
(PARI)
for(n=1, 10^8, if(n^2%(numdiv(n)*sigma(n))==0, print1(n, ", "))) \\ Derek Orr, Aug 08 2014
CROSSREFS
Sequence in context: A250972 A235096 A235900 * A034622 A004046 A031652
KEYWORD
nonn
AUTHOR
Jaroslav Krizek, Aug 08 2014
EXTENSIONS
a(7)-a(10) from Derek Orr, Aug 08 2014
a(11)-a(26) from Jens Kruse Andersen, Aug 13 2014
STATUS
approved

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Last modified July 22 07:42 EDT 2024. Contains 374481 sequences. (Running on oeis4.)