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%I #34 Nov 14 2018 22:10:48
%S 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,
%T 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,
%U 0,1,0,0,0,0,0,0,0,0,1,0,0,1,0,0,1,0,1
%N a(n) is the number of primes of the form p*10^n + q, where p, q are the digits from 1 to 9.
%H Robert Israel, <a href="/A321281/b321281.txt">Table of n, a(n) for n = 1..3000</a>
%H Sabin Tabirca and Kieran Reynolds, <a href="http://multimedia.ucc.ie/Staff/ST/articles/SNJ03_Tabirca1.ps">Lacunary Prime Numbers</a>.
%e 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.
%p f:= n -> nops(select(isprime,[seq(seq(p*10^n+q,p=1..9),q=[1,3,7, 9])])):
%p map(f, [$1..100]); # _Robert Israel_, Nov 14 2018
%t 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 *)
%o (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
%Y Cf. A000040.
%K nonn,base
%O 1,1
%A _Anton Deynega_, Nov 10 2018
%E a(16)-a(86) from _Andrew Howroyd_, Nov 10 2018
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