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A079155
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The number of primes less than 10^n whose digital root (A038194) is also prime.
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0
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OFFSET
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1,1
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LINKS
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EXAMPLE
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a(2) = 15 because the only primes less than 100 whose have digital roots are also prime are {2,3,5,7,11,23,29,41,43,47,59,61,79,83,97}.
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MATHEMATICA
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c = 0; k = 1; Do[ While[ k < 10^n, If[ PrimeQ[k] && PrimeQ[ Mod[k, 9]], c++ ]; k++ ]; Print[c], {n, 1, 8}]
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PROG
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(Python)
# use primerange (slower) vs. sieve.primerange (>> memory) for larger terms
from sympy import isprime, sieve
def afind(terms):
s = 0
for n in range(1, terms+1):
s += sum(isprime(p%9) for p in sieve.primerange(10**(n-1), 10**n))
print(s, end=", ")
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CROSSREFS
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KEYWORD
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base,nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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