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A096241
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Number of n-digit base-8 deletable primes.
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0
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4, 14, 50, 238, 1123, 5792, 30598, 166056, 927639, 5308458, 30984757
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OFFSET
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1,1
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COMMENTS
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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.
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LINKS
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MATHEMATICA
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b = 8; a = {4}; d = {2, 3, 5, 7};
For[n = 2, n <= 5, 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]];
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PROG
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(Python)
from sympy import isprime
def ok(n, prevset, base=8):
if not isprime(n): return False
s = oct(n)[2:]
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):
s, snxt = {2, 3, 5, 7}, set()
print(len(s), end=", ")
for n in range(2, terms+1):
for i in range(8**(n-1), 8**n):
if ok(i, s):
snxt.add(i)
s, snxt = snxt, set()
print(len(s), end=", ")
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CROSSREFS
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KEYWORD
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nonn,base,more
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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