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 A182394 Signs of differences of number of divisors function: a(n) = sign(d(n)-d(n-1)), cf. A000005. 4
 1, 0, 1, -1, 1, -1, 1, -1, 1, -1, 1, -1, 1, 0, 1, -1, 1, -1, 1, -1, 0, -1, 1, -1, 1, 0, 1, -1, 1, -1, 1, -1, 0, 0, 1, -1, 1, 0, 1, -1, 1, -1, 1, 0, -1, -1, 1, -1, 1, -1, 1, -1, 1, -1, 1, -1, 0, -1, 1, -1, 1, 1, 1, -1, 1, -1, 1, -1, 1, -1, 1, -1, 1, 1, 0, -1, 1 (list; graph; refs; listen; history; text; internal format)
 OFFSET 2 COMMENTS d(n) (A000005) has offset 1, being an arithmetic function, so this sequence has offset 2. Erdős proves that a(n) = 1 with natural density 1/2 and a(n) = -1 with natural density 1/2. Heath-Brown proved that a(n) = 0 infinitely often; see A005237 for details. - Charles R Greathouse IV, Oct 20 2013 LINKS N. J. A. Sloane, Table of n, a(n) for n = 2..20000 P. Erdős, On a problem of Chowla and some related problems, Proc. Cambridge Philos. Soc. 32 (1936), pp. 530-540. D. R. Heath-Brown, The divisor function at consecutive integers, Mathematika 31 (1984), pp. 141-149. FORMULA a(n) = 1 if d(n) > d(n - 1) and a(n) = -1 if d(n) < d(n - 1), otherwise a(n) = 0 if d(n) = d(n - 1), where d(n) is the number of divisors of n (A000005). EXAMPLE The initial values d(1) ... d(20) are 1, 2, 2, 3, 2, 4, 2, 4, 3, 4, 2, 6, 2, 4, 4, 5, 2, 6, 2, 6, ... and the first differences are 1, 0, 1, -1, 2, -2, 2, -1, 1, -2, 4, -4, 2, 0, 1, -3, 4, -4, 4, ..., the signs of which are +1, 0, +1, -1, ... MATHEMATICA Sign[Differences[DivisorSigma[0, Range[2..100]]]] (* T. D. Noe, Apr 27 2012, amended by N. J. A. Sloane, Oct 05 2017 *) PROG (PARI) a(n)=sign(numdiv(n)-numdiv(n-1)) \\ Charles R Greathouse IV, Oct 20 2013 CROSSREFS Cf. A000005, A051950, A175150 (accumulated sums). Sequence in context: A128973 A176412 A013596 * A079054 A131695 A324113 Adjacent sequences:  A182391 A182392 A182393 * A182395 A182396 A182397 KEYWORD sign,easy AUTHOR Giovanni Teofilatto, Apr 27 2012 EXTENSIONS Edited by N. J. A. Sloane, Oct 05 2017 STATUS approved

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Last modified April 6 18:51 EDT 2020. Contains 333286 sequences. (Running on oeis4.)