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A321281
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a(n) is the number of primes of the form p*10^n + q, where p, q are the digits from 1 to 9.
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1
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21, 15, 13, 8, 9, 5, 3, 8, 8, 2, 2, 3, 2, 0, 2, 2, 2, 3, 2, 5, 1, 4, 0, 3, 1, 1, 1, 2, 2, 0, 2, 0, 0, 0, 2, 2, 1, 1, 3, 1, 0, 2, 0, 0, 3, 2, 0, 0, 1, 0, 0, 1, 1, 0, 1, 2, 2, 1, 0, 2, 0, 1, 0, 0, 1, 0, 1, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 1, 0, 0, 1, 0, 1
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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(6) = 5 because there are five primes of the form p*10^6 + q where p, q are the digits from 1 to 9: 1000003, 2000003, 7000003, 7000009, 8000009.
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MAPLE
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f:= n -> nops(select(isprime, [seq(seq(p*10^n+q, p=1..9), q=[1, 3, 7, 9])])):
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MATHEMATICA
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a[n_]:=(c=0; Do[ Do[ If[PrimeQ[i*10^n+j], c++], {i, 1, 9}], {j, 1, 9, 2}]; c); Array[a, 20] (* Amiram Eldar, Nov 14 2018 *)
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PROG
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(PARI) a(n)={my(t=10^n); sum(i=1, 9, sum(j=1, 5, isprime(2*j-1+i*t)))} \\ Andrew Howroyd, Nov 10 2018
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CROSSREFS
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KEYWORD
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nonn,base
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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