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 A285893 Least number to start a run of exactly n nondecreasing values of sigma (sum of divisors, A000203). 6
 45, 5, 313, 1, 356067536821, 36721681 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS a(6) = 36721681, see also A028965. The analogous sequence based on tau = A000005 instead of sigma is A284597. LINKS EXAMPLE We have the following values of sigma for n = 1..10:          n   1   2   3   4   5   6   7   8   9  10  ...    sigma(n)  0   1   1   2   1   2   1   3   2   2  ... We see a run of 4 nondecreasing values starting at 1, ending at 4, therefore a(4) = 1. There is a run of 2 nondecreasing values starting at 5, ending at 6, therefore a(2) = 5. Correspondingly, a run of length 1 corresponds to a number n such that sigma(n-1) > sigma(n) > sigma(n+1). This happens first at a(1) = 45. MATHEMATICA Function[s, {45}~Join~Map[Function[r, Select[s, Last@ # == r &][[1, 1]]], Range[2, Max[s[[All, -1]] ] ]]]@ Map[{#[[1, 1]], Length@ # + 1} &, DeleteCases[SplitBy[#, #[[-1]] >= 0 &], k_ /; k[[1, -1]] < 0]] &@ MapIndexed[{First@ #2, #1} &, Differences@ Array[DivisorSigma[1, #] &, 10^6]] (* Michael De Vlieger, May 06 2017 *) PROG (PARI) alias(A, A285893); A=vector(19); apply(scan(N, s=1, t=sigma(s))=for(k=s+1, N, t>(t=sigma(k))||next; k-s>#A||A[k-s]||printf("a(%d)=%d, ", k-s, s)||A[k-s]=s; s=k); done, [10^8]) \\ Search may be extended using scan(END, START). CROSSREFS Cf. A000005, A000203, A028965, A284597 (analog for sigma_0), A286287 (analog for omega = A001221), A286288 (analog for bigomega = A001222). Sequence in context: A037940 A037218 A070818 * A129536 A108909 A033365 Adjacent sequences:  A285890 A285891 A285892 * A285894 A285895 A285896 KEYWORD nonn,more,hard AUTHOR M. F. Hasler, May 06 2017 EXTENSIONS a(5)-a(6) from Giovanni Resta, May 07 2017 STATUS approved

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Last modified January 28 06:29 EST 2020. Contains 331317 sequences. (Running on oeis4.)