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 A307959 Primitive coreful perfect numbers: powerful numbers k such that csigma(k) = 2*k, where csigma(k) is the sum of the coreful divisors of k (A057723). 8
 36, 392, 30752, 2064512, 549621604352, 2251765454077952, 144114638320566272, 9903520305059670164485701632, 12259964326927110856232952250923146488025299504439754752, 237142198758023568227473376531545064850552499416058362196624559881526665860349952 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS All the coreful perfect numbers (A307958) can be obtained from a primitive term k by multiplying it by m, if m is squarefree and coprime to k. The primitive terms are powerful. If k = m * r is in the sequence where r = rad(k) is the squarefree kernel of k (A007947), then the sum of the coreful divisors of k is csigma(k) = csigma(m * r) = sigma(m) * r, where sigma(m) is the sum of all the divisors of m (A000203). Thus k is a primitive coreful perfect number, iff sigma(m) * r = 2 * m * r, or m = k/rad(k) is a perfect number. Since k is powerful k = m * rad(m), thus all the primitive coreful perfect numbers can be generated from the perfect numbers by multiplying them by their squarefree kernel. The even terms of these sequence are (2^p) * (2^p-1)^2, were p is a Mersenne exponent (A000043). There is an odd term in this sequence iff there is an odd perfect number. LINKS FORMULA a(n) = A064549(A000396(n)) = A000396(n) * A007947(A000396(n)). EXAMPLE The first coreful perfect numbers are 36, 180 = 36*5, 252 = 36*7, 392, 396 = 36*11, 468 = 36*13, ... thus the primitive ones are 36, 392, ... MATHEMATICA f[p_] := 2^p*(2^p-1)^2; f/@MersennePrimeExponent/@Range[10] (* assuming no odd perfect number exists *) CROSSREFS Cf. A000043, A000203, A000396, A000668, A007947, A057723, A064549, A307958. Sequence in context: A254644 A034686 A160280 * A095682 A083811 A086575 Adjacent sequences: A307956 A307957 A307958 * A307960 A307961 A307962 KEYWORD nonn AUTHOR Amiram Eldar, May 08 2019 STATUS approved

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Last modified March 26 13:22 EDT 2023. Contains 361549 sequences. (Running on oeis4.)