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!)
A180132 Smallest k such that k*5^n is a sum of two successive primes. 9
5, 1, 4, 10, 2, 8, 12, 12, 36, 12, 28, 66, 30, 6, 18, 132, 36, 108, 34, 14, 48, 60, 12, 22, 150, 30, 6, 74, 54, 16, 8, 66, 150, 30, 6, 14, 374, 110, 22, 82, 62, 66, 108, 348, 114, 428, 190, 38, 570, 114, 102, 24, 82, 86, 178, 420, 84, 108, 328, 186, 126, 192, 76, 82, 24 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,1

COMMENTS

If a(n) == 0 (mod 5), then a(n+1) = a(n)/5.

Records: 5, 10, 12, 36, 66, 132, 150, 374, 428, 570, 734, 840, 1938, 2036, 2220, 2968, 3132, 3444, 4014, 6090, ..., .

Corresponding primes are twin primes for n = 0, 1, 51, 102, 103, 202, 275, ..., .

LINKS

Robert G. Wilson v, Table of n, a(n) for n = 0..500

MATHEMATICA

f[n_] := Block[{k = 1, j = 5^n/2}, While[ h = k*j; PrimeQ@h || NextPrime[h, -1] + NextPrime@h != 2 h, k++ ]; k]; Array[f, 80, 0]

PROG

(Python)

from sympy import nextprime, prevprime

def sum2succ(n): return n == prevprime(n//2) + nextprime(n//2)

def a(n):

  if n < 2: return [5, 1][n]

  k, pow5 = 1, 5**n

  while not sum2succ(k*pow5): k += 1

  return k

print([a(n) for n in range(65)]) # Michael S. Branicky, May 01 2021

CROSSREFS

Cf. A180130, A180131, A180133, A180134, A179975, A180135, A180136, A180137, A180138.

Sequence in context: A232809 A011301 A316248 * A286593 A242376 A307393

Adjacent sequences:  A180129 A180130 A180131 * A180133 A180134 A180135

KEYWORD

nonn

AUTHOR

Zak Seidov & Robert G. Wilson v, Aug 15 2010

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 June 19 18:35 EDT 2021. Contains 345144 sequences. (Running on oeis4.)