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A055521
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Restricted left truncatable (Henry VIII) primes.
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3
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773, 3373, 3947, 4643, 5113, 6397, 6967, 7937, 15647, 16823, 24373, 33547, 34337, 37643, 56983, 57853, 59743, 62383, 63347, 63617, 69337, 72467, 72617, 75653, 76367, 87643, 92683, 97883, 98317, 121997, 124337, 163853, 213613, 236653
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OFFSET
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1,1
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COMMENTS
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There are 1440 such primes, the largest being 357686312646216567629137.
Left-truncatable primes (A024785) which have at least two digits and are not the end of a larger left-truncatable prime. - Jens Kruse Andersen, Jul 29 2014
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REFERENCES
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Kahan, S. and Weintraub, S. "Left Truncatable Primes." J. Recr. Math. 29, 254-264, 1998.
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LINKS
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EXAMPLE
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773 is in the sequence since 773, 73, 3 are primes, while no digit 1..9 gives a prime if placed before 773. 13 is not in the sequence since for example 113 is prime. 2 and 5 are disqualified for only having one digit. - Jens Kruse Andersen, Jul 29 2014
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PROG
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(Python)
from sympy import isprime, primerange
def afull():
alst, prime_strs, an, digits = [], ["2", "3", "5", "7"], 0, 1
while len(prime_strs) > 0:
new_prime_strs = set()
for p in prime_strs:
can_extend = False
for d in "123456789":
c = d + p
if isprime(int(c)):
can_extend = True
new_prime_strs.add(c)
if digits > 1 and not can_extend:
alst.append(int(p))
prime_strs = new_prime_strs
digits += 1
return sorted(alst)
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CROSSREFS
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KEYWORD
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nonn,base,fini,full
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AUTHOR
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STATUS
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approved
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