OFFSET
0,26
COMMENTS
LINKS
Robert Israel, Table of n, a(n) for n = 0..2000
FORMULA
a(n) = n-A341701(n).
a(p) = 0 if and only if p is prime.
EXAMPLE
a(10) = 1 since 109 is prime. a(22) = 1 since 2221 is prime.
MAPLE
tcat:= proc(x, y) x*10^(1+ilog10(y))+y end proc:
f:= proc(n) local x, k;
x:= n;
for k from 0 to n-1 do
if isprime(x) then return k fi;
x:= tcat(x, n-k-1)
od;
-1
end proc:
map(f, [$0..100]); # Robert Israel, Mar 02 2022
PROG
(Python)
from sympy import isprime
def A341702(n):
k, m = n, n-1
while not isprime(k) and m > 0:
k = int(str(k)+str(m))
m -= 1
return n-m-1 if isprime(k) else -1
(PARI) a(n) = my(k=0, s=Str(n)); while (!isprime(eval(s)), k++; n--; if (k>=n, return(-1)); s = concat(s, Str(n-k))); return(k); \\ Michel Marcus, Mar 02 2022
CROSSREFS
KEYWORD
sign,base
AUTHOR
Chai Wah Wu, Feb 23 2021
STATUS
approved