login
The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A330879 Numbers with a divisor pair (d,n/d) such that the smallest prime greater than d and n/d is the same. 1
1, 4, 9, 12, 16, 25, 30, 36, 49, 56, 63, 64, 70, 72, 80, 81, 90, 100, 121, 132, 144, 169, 182, 195, 196, 208, 210, 224, 225, 240, 256, 289, 306, 324, 361, 380, 399, 400, 418, 420, 440, 441, 462, 484, 529, 552, 575, 576, 598, 600, 621, 624, 625, 644, 648, 650, 672 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

For n > 0, A000290(n) is a term. - Ivan N. Ianakiev, May 03 2020

For nonsquares, d is the tau(n)/2-th divisor of n. - David A. Corneth, May 03 2020

LINKS

David A. Corneth, Table of n, a(n) for n = 1..10000

EXAMPLE

9 is in the sequence since it has the divisor pair (3,3) with each divisor sharing the same next prime, which is 5.

12 is in the sequence since it has the divisor pair (3,4) and both 3 and 4 have 5 as their next prime.

MATHEMATICA

Table[If[Sum[KroneckerDelta[NextPrime[i], NextPrime[n/i]] (1 - Ceiling[n/i] + Floor[n/i]), {i, n}] > 0, n, {}], {n, 500}] // Flatten

PROG

(PARI) isok(n) = fordiv(n, d, if (nextprime(d+1) == nextprime(n/d+1), return (1)); if (d>n/d, break)); \\ Michel Marcus, Apr 30 2020

CROSSREFS

Cf. A056737, A151800.

Sequence in context: A034019 A034018 A320924 * A285419 A047461 A276873

Adjacent sequences:  A330876 A330877 A330878 * A330880 A330881 A330882

KEYWORD

nonn

AUTHOR

Wesley Ivan Hurt, Apr 30 2020

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 July 26 01:49 EDT 2021. Contains 346294 sequences. (Running on oeis4.)