A096237 Number of n-digit base-4 deletable primes.

2, 3, 9, 26, 75, 213, 615, 1853, 5854, 18664, 61248, 205300, 698575, 2409598, 8408050, 29657194

Offset

1,1

Comments

A prime p is a base-b deletable prime if when written in base b it has the property that removing some digit leaves either the empty string or another deletable prime. "Digit" means digit in base b.

Deleting a digit cannot leave any leading zeros in the new string. For example, deleting the 2 in 2003 to obtain 003 is not allowed.

Links

Table of n, a(n) for n=1..16.

Mathematica

b = 4; a = {2}; d = {2, 3};
For[n = 2, n <= 8, n++,
  p = Select[Range[b^(n - 1), b^n - 1], PrimeQ[#] &];
  ct = 0;
  For[i = 1, i <= Length[p], i++,
   c = IntegerDigits[p[[i]], b];
   For[j = 1, j <= n, j++,
    t = Delete[c, j];
    If[t[[1]] == 0, Continue[]];
    If[MemberQ[d, FromDigits[t, b]], AppendTo[d, p[[i]]]; ct++;
     Break[]]]];
  AppendTo[a, ct]];
a (* Robert Price, Nov 12 2018 *)

Prog

(Python)
from sympy import isprime
from sympy.ntheory.digits import digits
def ok(n, prevset, base=4):
    if not isprime(n): return False
    s = "".join(str(d) for d in digits(n, base)[1:])
    si = (s[:i]+s[i+1:] for i in range(len(s)))
    return any(t[0] != '0' and int(t, base) in prevset for t in si)
def afind(terms):
    alst = [2]
    s, snxt, base = {2, 3}, set(), 4
    print(len(s), end=", ")
    for n in range(2, terms+1):
        for i in range(base**(n-1), base**n):
            if ok(i, s):
                snxt.add(i)
        s, snxt = snxt, set()
        print(len(s), end=", ")
afind(10) # Michael S. Branicky, Jan 17 2022

Crossrefs

Cf. A080608, A080603, A096235-A096246.

Sequence in context: A038523 A358410 A361863 * A309814 A177928 A057231

Adjacent sequences: A096234 A096235 A096236 * A096238 A096239 A096240

Keyword

nonn,base,more

Author

Michael Kleber, Feb 28 2003

Extensions

a(6)-a(15) from Ryan Propper, Jul 19 2005

a(16) from Michael S. Branicky, Jan 17 2022

Status

approved