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 A229238 Numbers k such that phi(sigma(k))/sigma(phi(k)) = 2. 3
 2, 4, 16, 18, 64, 100, 450, 1458, 4096, 4624, 28900, 36450, 62500, 65536, 130050, 262144, 281250, 1062882, 1336336, 3334800, 7064400, 8352100, 10156800, 10534050, 18062500, 21193200, 22781250, 26572050, 37584450, 39062500, 48944016, 81281250, 124411716 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS 2^j is in the sequence if and only if 2^{j+1}-1 is a Mersenne prime. In other words 2^j is the "even part" of a perfect number. Thus we have some generalization of perfect numbers. Odd prime divisors of the first 19 terms of a(n) are exclusively 3, 5, 17, i.e., Fermat's primes, but 3334800 = 2^4*3*5^2*7*397. LINKS Donovan Johnson, Table of n, a(n) for n = 1..73 EXAMPLE 18 is in the sequence because phi(sigma(18)) = phi(39) = 24 = 2*sigma(6) = 2*sigma(phi(18)). MAPLE s:=n->phi(sigma(n))/sigma(phi(n)); for i to 9000000 do if s(i)=2 then print(i) fi od: PROG (PARI) isok(n) = (eulerphi(sigma(n)) == 2*sigma(eulerphi(n))); \\ Michel Marcus, Sep 23 2013 CROSSREFS Cf. A000010, A000203, A033632. Sequence in context: A186108 A131560 A067709 * A212202 A102545 A045521 Adjacent sequences: A229235 A229236 A229237 * A229239 A229240 A229241 KEYWORD nonn AUTHOR Vladimir Letsko, Sep 17 2013 EXTENSIONS Extra term 4624 and more terms from Michel Marcus, Sep 23 2013 STATUS approved

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Last modified September 30 20:58 EDT 2023. Contains 365796 sequences. (Running on oeis4.)