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%I
%S 4,15,85,619,4800,39266,332276,2880818
%N The number of primes less than 10^n whose digital root (A038194) is also prime
%e 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}.
%t c = 0; k = 1; Do[ While[ k < 10^n, If[ PrimeQ[k] && PrimeQ[ Mod[k, 9]], c++ ]; k++ ]; Print[c], {n, 1, 8}]
%Y The primes are in A078403, their digital roots are in A078400.
%K base,nonn
%O 1,1
%A _Robert G. Wilson v_, Dec 27 2002
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