This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A153328 #21 Dec 11 2024 17:56:52 %S A153328 6,10,42,138,626,1398,1596,1978,7808,14058,46498 %N A153328 Numbers k such that (10^k-1)*120/99 + 1 is prime. %C A153328 Also 2*A056803 which I took the liberty of using to create the last 2 entries. %C A153328 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 A153328 (PARI) /* n=number of values to test; r=repeat digits, e.g., 12, 121, 177, 1234; d = last digit appended to the end */ %o A153328 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 A153328 Cf. A092696, A056803. %K A153328 nonn,base,more %O A153328 1,1 %A A153328 _Cino Hilliard_, Dec 23 2008 %E A153328 a(11) from _Michael S. Branicky_, Dec 11 2024