OFFSET
2,3
COMMENTS
Prime digits p in base n are counted if there is no prime with 2 digits which can have its rightmost digit removed to produce p.
LINKS
I. O. Angell and H. J. Godwin, On Truncatable Primes, Math. Comput. 31, 265-267, 1977.
Eric Weisstein's World of Mathematics, Truncatable Prime.
PROG
(Python)
from sympy import isprime, primerange
def fromdigits(digs, base):
return sum(d*base**i for i, d in enumerate(digs))
def a(n):
prime_lists, an = [(p, ) for p in primerange(1, n)], 0
digits = 1
while len(prime_lists) > 0:
new_prime_strs = set()
for p in prime_lists:
can_extend = False
for d in range(n):
c = (d, ) + p
if isprime(fromdigits(c, n)):
can_extend = True
new_prime_strs.add(c)
if not can_extend:
an += 1
prime_lists = list(new_prime_strs)
digits += 1
return an
print([a(n) for n in range(2, 27)]) # Michael S. Branicky, Dec 11 2022
CROSSREFS
KEYWORD
nonn
AUTHOR
Martin Renner, Jan 04 2008
EXTENSIONS
a(6) corrected and a(11) and beyond from Michael S. Branicky, Dec 11 2022
STATUS
approved