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A076774
2-nadirs of sigma: numbers k such that sigma(k-2) > sigma(k-1) > sigma(k) < sigma(k+1) < sigma(k+2).
1
17254, 27754, 68914, 69686, 82774, 92366, 111266, 133706, 152426, 194426, 267994, 277586, 359974, 387694, 389726, 429274, 448174, 452726, 457766, 471626, 474146, 522026, 527066, 531334, 554126, 567386, 595594, 610226, 674246, 674974
OFFSET
1,1
COMMENTS
I call n a "k-nadir" (or nadir of depth k) of the arithmetical function f if n satisfies f(n-k) > ... > f(n-1) > f(n) < f(n+1) < ... < f(n+k).
LINKS
MATHEMATICA
Select[Range[3, 10^6], DivisorSigma[1, # - 2] > DivisorSigma[1, # - 1] > DivisorSigma[1, # ] < DivisorSigma[1, # + 1] < DivisorSigma[1, # + 2] &]
Flatten[Position[Partition[DivisorSigma[1, Range[675000]], 5, 1], _? (#[[1]]> #[[2]]>#[[3]]<#[[4]]<#[[5]]&), 1, Heads->False]]+2 (* Harvey P. Dale, Jan 04 2022 *)
PROG
(Magma) ds:=DivisorSigma; f:=func<n|ds(1, n) lt ds(1, n+1) and ds(1, n+1) lt ds(1, n+2)>; f1:= func<n|ds(1, n) lt ds(1, n-1) and ds(1, n-1) lt ds(1, n-2)>; [k:k in [3..675000]|f(k) and f1(k)]; // Marius A. Burtea, Feb 19 2020
CROSSREFS
Cf. A000203.
Sequence in context: A043621 A334310 A293478 * A236447 A094413 A209967
KEYWORD
nonn
AUTHOR
Joseph L. Pe, Nov 14 2002
STATUS
approved