login
The OEIS is supported by the many generous donors to the OEIS Foundation.

 

Logo
Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A075027 Numbers k such that d(k-1) < d(k) > d(k+1), d = A000005. 5
4, 6, 8, 10, 12, 16, 18, 20, 24, 28, 30, 32, 36, 40, 42, 48, 50, 52, 54, 56, 60, 64, 66, 68, 70, 72, 78, 80, 84, 88, 90, 92, 96, 100, 102, 108, 110, 112, 114, 120, 124, 126, 128, 130, 132, 138, 140, 144, 150, 152, 154, 156, 160, 162, 165, 168, 170, 174, 176, 180 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Obviously every term is composite.

The average of each twin prime pair is a term.

a(55) = 165 is the first odd term; A323379 lists all odd terms. - Jon E. Schoenfield, Sep 26 2021

LINKS

Michael De Vlieger, Table of n, a(n) for n = 1..3403 (first 1001 terms from Harvey P. Dale)

EXAMPLE

10 is a term since d(9) = 3, d(10) = 4, d(11) = 2 and 3 < 4 > 2.

MAPLE

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

select(q, [$1..200])[];  # Alois P. Heinz, Sep 28 2021

MATHEMATICA

#[[2, 1]]&/@Select[Partition[Table[{n, DivisorSigma[0, n]}, {n, 200}], 3, 1], #[[1, -1]]<#[[2, -1]]>#[[3, -1]]&] (* Harvey P. Dale, Oct 09 2011 *)

CROSSREFS

Cf. A000005, A014574, A075025, A075026, A323379.

Sequence in context: A322839 A074827 A068354 * A309837 A310659 A076298

Adjacent sequences:  A075024 A075025 A075026 * A075028 A075029 A075030

KEYWORD

nonn

AUTHOR

Amarnath Murthy, Sep 02 2002

EXTENSIONS

More terms from Jason Earls, Sep 04 2002

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified January 21 15:30 EST 2022. Contains 350479 sequences. (Running on oeis4.)