OFFSET
1,2
COMMENTS
Is it a theorem that a(n) aways exists?
Yes: as long as nonzero digits are used, eventually you reach a number x starting with 10, large enough that there is a prime between x and 3*x/2. All the numbers from x to 3*x/2 start with 1, so if you use the digit 1 you will eventually reach a prime. - Robert Israel, Mar 17 2019
A variant of this (A241181) sets a(n) = 0 if n is already a prime.
REFERENCES
Eric Angelini, Posting to Sequence Fans Mailing List, Apr 20 2014
LINKS
Hiroaki Yamanouchi, Table of n, a(n) for n = 1..100000
EXAMPLE
Examples, in condensed notation:
1+1=2
2+2=4+4=8+8=16+1=17
3+3=6+6=12+1=13
4+4=8+8=16+1=17
5+5=10+1=11
6+6=12+1=13
7+7=14+4=18+1=19
8+8=16+1=17
9+9=18+1=19
10+1=11
11+1=12+1=13
12+1=13
13+3=16+1=17
14+4=18+1=19
15+1=16+1=17
16+1=17
17+1=18+1=19
18+1=19
19+9=28+8=36+3=39+9=48+8=56+5=61
20+2=22+2=24+2=26+6=32+2=34+3=37
...
MAPLE
g:= proc(n, Nmax) option remember; local L, d, t;
if isprime(n) then return 0 fi;
if n > Nmax then return infinity fi;
L:= convert(convert(n, base, 10), set) minus {0};
1 + min(seq(procname(n+d), d=L));
end proc:
f:= proc(n, Nmax) local L, d, t;
L:= convert(convert(n, base, 10), set) minus {0};
1 + min(seq(g(n+d, Nmax), d=L))
end proc:
map(f, [$1..200], 1000); # Robert Israel, Mar 17 2019
MATHEMATICA
A241180[n_] := Module[{c, nx},
c = 1; nx = n;
While[ !
AnyTrue[nx = Flatten[nx + IntegerDigits[nx]],
PrimeQ [#] && # > n &], c++];
Return[c]];
Table[A241180[i], {i, 100}] (* Robert Price, Mar 17 2019 *)
CROSSREFS
KEYWORD
easy,nonn,base
AUTHOR
N. J. A. Sloane, Apr 23 2014
EXTENSIONS
a(23)-a(87) from Hiroaki Yamanouchi, Sep 05 2014
STATUS
approved