%I #17 Dec 12 2021 01:19:04
%S 6,10,42,138,626,1398,1596,1978,7808,14058
%N Numbers k such that (10^k-1)*120/99+1 is prime.
%C Also 2*A056803 which I took the liberty of using to create the last 2 entries.
%C These numbers are always even. If n is odd, then 10^n-1 produces a number with an odd number of 9's which 99 does not divide. a(6), a(10), a(42) are 1212121, 12121212121, 1212121212121212121212121212121212121212121 which can be found in A092696. Also, the formula produce palindromic numbers.
%o (PARI) /* n=number of values to test; r=repeat digits, e.g., 12, 121, 177, 1234; d = last digit appended to the end */
%o repr(n,r,d) = ln=length(Str(r));for(x=0,n,y=(10^(ln*x)-1)*10*r/ (10^ln-1)+1;if(ispseudoprime(y),print1(ln*x",")))
%Y Cf. A092696, A056803.
%K nonn,base,more
%O 1,1
%A _Cino Hilliard_, Dec 23 2008
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