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%I #19 May 25 2016 08:27:40
%S 4,4,15,38,128,389,1325,4643,16623,59241,214432,781471,2884201,
%T 10687480,39838489,149172297,560795383,2115430020
%N Number of n-digit primes whose digits are all primes.
%C Number of primes of the form: ...d3d2d1d0 = d0 * 10 ^ 0 + d1 * 10 ^ 1 + d2 * 10 ^ 2 + d3 * 10 ^ 3 + ... where d0, d1, d2, d3, ... are primes with one digit. The i-th element of the sequence is the number of primes with i digits. Approximation of the sum of the sequence up to a(k) for large values of k: Sum_{i=1..k} a(i) = 10^k / (k*log(10))*(2/5)^(k-1).
%e a(4) = 38 because there are 38 numbers of the form d3d2d1d0 with d0, d1, d2, d3 prime numbers, namely, 2237, 2273, 2333, 2357, 2377, 2557, ..., 7753, 7757.
%Y Cf. A135945, A135946.
%K nonn,base,more
%O 1,1
%A _Giorgio Balzarotti_ & _Paolo P. Lava_, Dec 07 2007
%E a(12)-a(14) from _Donovan Johnson_, Feb 05 2010
%E a(15)-a(16) from _Chai Wah Wu_, Nov 28 2015
%E a(17)-a(18) from _Giovanni Resta_, May 25 2016
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