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 A335254 Numbers k such that the abundance (A033880) of k is equal to the deficiency (A033879) of k+1. 3
 672, 523776, 19327369215 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Equivalently, k and k+1 have the same absolute value of abundance (or deficiency) with opposite signs. Equivalently, s(k) + s(k+1) = k + (k+1), where s(k) is the sum of proper divisors of k (A001065). If k is a 3-perfect number (A005820) and k+1 is a prime, then k is in the sequence. Of the 6 known 3-perfect numbers only 672 and 523776 have this property. a(4) > 10^11, if it exists. a(4) > 10^13, if it exists. - Giovanni Resta, May 30 2020 LINKS EXAMPLE 672 is a term since A033880(672) = sigma(672) - 2*672 = 2016 - 1344 = 672, and A033879(673) = 2*673 - sigma(673) = 1346 - 674 = 672. MATHEMATICA ab[n_] := DivisorSigma[1, n] - 2*n; Select[Range[6 * 10^5], ab[#] == -ab[# + 1] &] CROSSREFS Cf. A000203, A001065, A005820, A033879, A033880, A330901, A335253. Sequence in context: A245782 A047728 A297123 * A160209 A234117 A171266 Adjacent sequences:  A335251 A335252 A335253 * A335255 A335256 A335257 KEYWORD nonn,hard,bref,more AUTHOR Amiram Eldar, May 28 2020 STATUS approved

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Last modified September 22 06:01 EDT 2021. Contains 347605 sequences. (Running on oeis4.)