OFFSET
0,1
COMMENTS
If a(n) == 0 (mod 7), then a(n+1) = a(n)/7.
Records: 5, 6, 18, 30, 60, 120, 186, 240, 286, 422, 536, 564, 798, 1010, 1074, 1334, 1434, 1474, 3706, 4108, 4370, 6160, ..., .
Corresponding prime are twin primes for n = 0, 17, 369, ..., .
LINKS
Robert G. Wilson v, Table of n, a(n) for n = 0..400
MATHEMATICA
f[n_] := Block[{k = 1, j = 7^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 isprime, nextprime, prevprime
def ok(n):
if n <= 5: return n == 5
return not isprime(n//2) and n == prevprime(n//2) + nextprime(n//2)
def a(n):
k, pow7 = 1, 7**n
while not ok(k*pow7): k += 1
return k
print([a(n) for n in range(64)]) # Michael S. Branicky, May 06 2021
CROSSREFS
KEYWORD
nonn
AUTHOR
Zak Seidov & Robert G. Wilson v, Aug 15 2010
STATUS
approved