A158962 a(n) is the smallest number m such that all n numbers m-1, m.m-1, ..., m.m. .. .m-1 are prime, where dot means concatenation.

3, 4, 4, 444, 827868, 10387692, 1885781034

Offset

1,1

Links

Table of n, a(n) for n=1..7.

Example

All 4 numbers 444-1,444444-1,1+444444444-1,444444444444-1 are prime and 444
is the smallest such number so a(4)=444.

Maple

a:= proc(n) local m; for m do if andmap(isprime,
      [seq(parse(cat(m$i))-1, i=1..n)]) then return m fi od
    end:
seq(a(n), n=1..5);  # Alois P. Heinz, Apr 14 2021

Prog

(Python)
from sympy import isprime
def A158962(n):
    m = 1
    while True:
        for i in range(n):
            if not isprime(int(str(m)*(i+1))-1):
                break
        else:
            return m
        m += 1 # Chai Wah Wu, Apr 14 2021

Crossrefs

Cf. A153434.

Sequence in context: A067137 A060115 A100649 * A263824 A215747 A246667

Adjacent sequences: A158959 A158960 A158961 * A158963 A158964 A158965

Keyword

base,more,nonn

Author

Farideh Firoozbakht, Apr 02 2009

Status

approved