OFFSET
1,1
COMMENTS
If we consider the product instead of the sum, n = concat(s,t) = prime(s) * prime(t), then the first terms are 14 and 2127. In fact:
14 = concat(1,4) and prime(1) * prime(4) = 2 * 7 = 14.
2127 = concat(2,127) and prime(2) * prime(127) = 3 * 709 = 2127.
a(18) > 8*10^10. - Giovanni Resta, May 26 2015
EXAMPLE
254 = concat(2,54) and prime(2) + prime(54) = 3 + 251 = 254.
64581 = concat(6458,1) and prime(6458) + prime(1) = 64579 + 2 = 64581.
64582 = concat(6458,2) and prime(6458) + prime(2) = 64579 + 3 = 64582. Etc.
MAPLE
with(numtheory):P:=proc(q) local s, t, k, n;
for n from 1 to q do for k from 1 to ilog10(n) do s:=n mod 10^k; t:=trunc(n/10^k); if s*t>0 then if ithprime(s)+ithprime(t)=n
then print(n); break; fi; fi; od; od; end: P(10^6);
# program from R. J. Mathar, Jan 22 2015:
isA251860 := proc(n)
local ti, i1, i2;
if n >= 10 then
for ti from 1 to A055642(n)-1 do
i1 := modp(n, 10^ti) ;
i2 := floor(n/10^ti) ;
if i1 > 0 and i2 > 0 then
if ithprime(i1)+ithprime(i2) = n then
return true;
end if;
end if;
end do:
false;
else
false;
end if;
end proc:
for n from 1 do
if isA251860(n) then
print(n);
end if;
end do:
PROG
(PARI) isok(n) = {my(nb = #Str(n)); for (k=1, nb-1, s = n\10^k; t = n % 10^k; if (s && t && prime(s)+ prime(t) == n, return (1)); ); return (0); } \\ Michel Marcus, Dec 10 2014
CROSSREFS
KEYWORD
nonn,base,more
AUTHOR
Paolo P. Lava, Dec 10 2014
EXTENSIONS
a(13)-a(17) from Giovanni Resta, May 26 2015
STATUS
approved