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A075026 Define a number k to occupy a divisor cavity if d(k-1) > d(k) < d(k+1) where d(k) is the number of divisors of k. Sequence gives composite numbers occupying a divisor cavity. 3
9, 25, 49, 51, 55, 65, 69, 77, 91, 111, 115, 121, 125, 129, 153, 155, 161, 169, 175, 183, 185, 187, 209, 221, 235, 237, 247, 249, 259, 265, 267, 274, 287, 289, 291, 295, 305, 309, 319, 321, 323, 329, 339, 341, 343, 351, 355, 361, 365, 369, 371, 377, 386, 391 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,1

LINKS

Table of n, a(n) for n=0..53.

MAPLE

q:= k-> not isprime(k) and (d->

d(k-1)>d(k) and d(k)<d(k+1))(numtheory[tau]):

select(q, [$1..400])[]; # Alois P. Heinz, Sep 28 2021

MATHEMATICA

Select[Flatten[Position[Partition[DivisorSigma[0, Range[400]], 3, 1], _?(#[[1]]> #[[2]]<#[[3]]&), 1, Heads->False]]+1, CompositeQ] (* Harvey P. Dale, Oct 23 2019 *)

CROSSREFS

Cf. A000005, A075025, A075027.

Sequence in context: A291259 A051132 A247687 * A339727 A339128 A113659

Adjacent sequences: A075023 A075024 A075025 * A075027 A075028 A075029

KEYWORD

nonn

AUTHOR

Amarnath Murthy, Sep 02 2002

EXTENSIONS

Corrected and extended by Jason Earls, Sep 04 2002

STATUS

approved

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Last modified January 31 06:02 EST 2023. Contains 359947 sequences. (Running on oeis4.)