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 A129784 a(n) = floor(log_10(2^(2^n))). 0
 0, 1, 2, 4, 9, 19, 38, 77, 154, 308, 616, 1233, 2466, 4932, 9864, 19728, 39456, 78913, 157826, 315652, 631305, 1262611, 2525222, 5050445, 10100890, 20201781, 40403562, 80807124, 161614248, 323228496, 646456993, 1292913986, 2585827972 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS Starting with 2, n successive squarings yields an (a(n)+1)-digit number. Dubickas proves that infinitely many terms of this sequence are divisible by 2 or 3 (and hence infinitely many composites). - Charles R Greathouse IV, Feb 04 2016 REFERENCES ArtÅ«ras Dubickas, Prime and composite integers close to powers of a number, Monatsh. Math. 158:3 (2009), pp. 271-284. LINKS EXAMPLE a(16) = 19728 because floor(log_10(2^(2^16))) = floor(log_10(2^65536)) = floor(log_10(2.003529930406846*10^19728)) = floor(19728.30179583467) = 19728. MATHEMATICA Table[Floor[Log[10, 2^(2^n)]], {n, 1, 29}] (* Vincenzo Librandi, Dec 30 2015 *) PROG (MAGMA) [Floor(Log(10, 2^(2^n))): n in [1..29]]; // Vincenzo Librandi, Dec 30 2015 (PARI) a(n) = floor(log(2^(2^n))/log(10)); \\ Michel Marcus, Dec 30 2015 (PARI) a(n)=logint(2^2^n, 10) \\ impractical except for small n, but avoids rounding; Charles R Greathouse IV, Feb 04 2016 CROSSREFS Cf. A001146. Sequence in context: A292478 A309267 A262864 * A329356 A125050 A056186 Adjacent sequences:  A129781 A129782 A129783 * A129785 A129786 A129787 KEYWORD nonn AUTHOR Jon E. Schoenfield, May 18 2007 STATUS approved

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Last modified December 14 12:04 EST 2019. Contains 329979 sequences. (Running on oeis4.)