A065851 Let u be any string of n digits from {0,...,9}; let f(u) = number of distinct primes, not beginning with 0, formed by permuting the digits of u; then a(n) = max_u f(u).

1, 2, 4, 11, 39, 148, 731, 4333, 26519, 152526, 1251724

Offset

1,2

Comments

From David A. Corneth, Jan 25 2022: (Start)

a(12) >= 7627713. a(13) >= 49545041.

There are A006879(12) = 33489857205 primes with 12 digits. There are 2.4*10^11 numbers with 12 digits that are coprime to 30 (i.e. end in 1, 3, 7, 9 and have a digital sum not divisible by 3).

On average one in 2.4*10^11/33489857205 ~= 7.166 of these numbers with 12 digits is prime.

Let maxqprimes(d) be the number of permutations of digits of d without leading 0 and ending in 1, 3, 7 or 9.

Let qprimes(d) be the number of permutations of digits of d (without leading 0 and ending in 1, 3, 7 or 9) that are prime.

a(12) = 7627713 if for some number with 12 digits we have maxprimes(d)/qprimes(d) < 5 (found via a brute force check). This is much less than the expected 7.166.

(At least) 101233456789 with 12 digits has maxqprimes(101233456789) = 54432000. This d = 101233456789 has a shared record for largest value for maxqprimes(d), along with d in {101234567789, 101234567899, 102334567789, 102345677899}. This gives maxqprimes(101233456789)/qprimes(101233456789) ~= 7.136, close to the expected 7.166.

The values d that have maxqprimes(d) >= 5*7627713 all have maxqprimes(d)/qprimes(d) >= 7.13.

For a(12) there are 2.4*10^11 checks needed if we do not use the found bound of 7627713. If we do we can bring down the search space by a factor of about 4 (without any heuristic). If we increase to 7*7627713 we only have about 9*10^8 needed checks left to do. (End)

Links

Table of n, a(n) for n=1..11.

Example

a(2) = 2 because 13 and 31 are primes.
a(3) = 4 because {149, 419, 491, 941} or {179, 197, 719, 971} or {379, 397, 739, 937} are primes. - R. J. Mathar, Apr 23 2016

Mathematica

c[x_] := Module[{},
   Length[Select[Permutations[x],
     First[#] != 0 && PrimeQ[FromDigits[#, 10]] &]]];
A065851[n_] := Module[{i},
   Return[Max[Map[c, DeleteDuplicatesBy[Tuples[Range[0, 9], n],
       Table[Count[#, i], {i, 0, 9}] &]]]]];
Table[A065851[n], {n, 1, 6}] (* Robert Price, Mar 30 2019 *)

Prog

(Python)
from sympy import isprime
from itertools import combinations_with_replacement
from sympy.utilities.iterables import multiset_permutations
def a(n): return max(sum(1 for p in multiset_permutations(u) if p[0] != "0" and isprime(int("".join(p)))) for u in combinations_with_replacement("0123456789", n) if sum(int(ui) for ui in u)%3)
print([a(n) for n in range(1, 7)]) # Michael S. Branicky, Jan 24 2022

Crossrefs

Cf. A006879, A065843, A065844, A065845, A065846, A065847, A065848, A065849, A065850, A065852, A065853.

Sequence in context: A101898 A193188 A216810 * A173938 A242790 A013044

Adjacent sequences: A065848 A065849 A065850 * A065852 A065853 A065854

Keyword

nonn,base,more

Author

Sascha Kurz, Nov 24 2001

Extensions

1 more term from Sean A. Irvine, Sep 06 2009

Definition corrected by David A. Corneth, Apr 23 2016

a(11) from David A. Corneth, Jan 25 2022

Status

approved