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 A330606 Numbers k such that k*d(k) and sigma(k) are relatively prime, where d(k) is the number of divisors of k (A000005) and sigma(k) is their sum (A000203). 2
 1, 2, 4, 8, 9, 16, 25, 36, 64, 81, 100, 121, 128, 144, 225, 256, 289, 324, 400, 484, 512, 529, 576, 625, 729, 841, 900, 1024, 1089, 1156, 1250, 1296, 1600, 1681, 1936, 2025, 2116, 2209, 2304, 2401, 2500, 2601, 2809, 3025, 3364, 3481, 3600, 4096, 4356, 4624, 4761 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS If p is prime and p == 2 (mod 3) then p^2 is in the sequence. Let E(x) = #{n | a(n) <= x} be the number of terms of this sequence up to x. Kanold proved that there are two constants 0 < c1 < c2 and a positive number x_0 such that c1 < E(x)/sqrt(x/log(x)) < c2 for x > x_0. De Koninck and Kátai proved that there is a positive constant c such that E(x) = c * (1 + o(1)) * sqrt(x/log(x)). Apparently most of the terms are squares or powers of 2. Terms that are not included 1250, 4802, 31250, 57122, ... REFERENCES József Sándor, Dragoslav S. Mitrinovic, Borislav Crstici, Handbook of Number Theory I, Springer Science & Business Media, 2005, page 75. LINKS Amiram Eldar, Table of n, a(n) for n = 1..10000 Jean-Marie De Koninck and Imre Kátai, On an estimate of Kanold, Int. J. Math. Anal., Vol. 5, No. 8 (2007), pp. 1-12. Hans-Joachim Kanold, Über das harmonische Mittel der Teiler einer natürlichen Zahl II, Mathematische Annalen, Vol. 134, No. 3 (1958), pp. 225-231. MATHEMATICA Select[Range[10^4], CoprimeQ[# * DivisorSigma[0, #], DivisorSigma[1, #]] &] PROG (MAGMA) [k:k in [1..5000]| Gcd(k*NumberOfDivisors(k), DivisorSigma(1, k)) eq 1]; // Marius A. Burtea, Dec 20 2019 CROSSREFS Cf. A000005, A000203, A038040, A324121. Subsequence of A014567 and A046678. Sequence in context: A204826 A241010 A046678 * A046680 A331992 A113570 Adjacent sequences:  A330603 A330604 A330605 * A330607 A330608 A330609 KEYWORD nonn AUTHOR Amiram Eldar, Dec 20 2019 STATUS approved

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Last modified September 17 03:42 EDT 2021. Contains 347478 sequences. (Running on oeis4.)