A354456 a(n) is the least number k such that k - 5^i is prime for i = 1..n.

7, 28, 132, 666, 3234, 17514, 100674, 501228, 2062662, 211097334, 2597411082, 34473310284, 214852200444, 394471192794

Offset

1,1

Comments

a(15) > 5*10^12. - David A. Corneth, May 30 2022

Links

Table of n, a(n) for n=1..14.

Example

a(3) = 132 because 132 - 5^1 = 127, 132 - 5^2 = 107 and 132 - 5^3 = 7 are all prime, and 132 is the least number with this property.

Maple

g:= proc(n) local p, x, i;
   p:= 1:
   do
     p:= nextprime(p);
     x:= p + 5^n;
     if andmap(isprime, [seq(x-5^i, i=1..n-1)]) then return x fi
   od
end proc:
map(g, [$1..10]);

Prog

(Python)
from sympy import isprime, nextprime
def a(n):
    p = 2
    while True:
        k, p = 5**n + p, nextprime(p)
        if all(isprime(k-5**i) for i in range(1, n)):
            return k
print([a(n) for n in range(1, 10)]) # Michael S. Branicky, May 30 2022

Crossrefs

Cf. A000351, A175222.

Sequence in context: A219737 A316106 A359203 * A249872 A238448 A290356

Adjacent sequences: A354453 A354454 A354455 * A354457 A354458 A354459

Keyword

nonn,more

Author

J. M. Bergot and Robert Israel, May 30 2022

Extensions

a(11)-a(14) from David A. Corneth, May 30 2022

Status

approved