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 A045972 a(1)=9; if n = Product p_i^e_i, n > 1, then a(n) = Product p_{i+2}^{e_i+1}. 5
 9, 25, 49, 125, 121, 1225, 169, 625, 343, 3025, 289, 6125, 361, 4225, 5929, 3125, 529, 8575, 841, 15125, 8281, 7225, 961, 30625, 1331, 9025, 2401, 21125, 1369, 148225, 1681, 15625, 14161, 13225, 20449, 42875, 1849, 21025, 17689, 75625, 2209, 207025 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 REFERENCES From a puzzle proposed by Marc LeBrun. LINKS Table of n, a(n) for n=1..42. FORMULA Sum_{n>=1} 1/a(n) = (4/7) * (zeta(2)*zeta(3)/zeta(6)) - 8/9 = 0.221737646437... . - Amiram Eldar, Sep 19 2023 MATHEMATICA f[p_, e_] := NextPrime[p, 2]^(e + 1); a[1] = 9; a[n_] := Times @@ f @@@ FactorInteger[n]; Array[a, 100] (* Amiram Eldar, Sep 19 2023 *) CROSSREFS Cf. A045965, A045966, A045967, A045968, A045969, A045970, A045971, A045973, A082695. Sequence in context: A110588 A282631 A245694 * A297596 A266132 A112629 Adjacent sequences: A045969 A045970 A045971 * A045973 A045974 A045975 KEYWORD easy,nonn AUTHOR N. J. A. Sloane EXTENSIONS More terms from David W. Wilson STATUS approved

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Last modified August 5 01:22 EDT 2024. Contains 374934 sequences. (Running on oeis4.)