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 A107648 #36 Aug 03 2024 18:58:38
%S A107648 1,4,6,7,384,666,675,3165,131020
%N A107648 Numbers n such that (10^(2n+1)+63*10^n-1)/9 is prime.
%C A107648 n is in the sequence iff the palindromic number 1(n).8.1(n) is prime (dot between numbers means concatenation). Let f(n)=(10^(2n+1)+63*10^n-1)/9 then for all nonnegative integers m we have: I. 3 divides f(3m+2) II. 19 divides f(18m+13) III. 29 divides f(28*m+16) & 29 divides f(28*m+25) IV. 31 divides f(30*m+2) & 31 divides f(30*m+17) V. 41 divides f(5m+3), etc. So if n is in the sequence then n is not of the forms 3m+2, 18m+13, 28m+16 28m+25, 30m+2, 30m+17, 5m+3, etc.
%C A107648 a(9) > 10^5. - _Robert Price_, Oct 30 2017
%D A107648 C. Caldwell and H. Dubner, "Journal of Recreational Mathematics", Volume 28, No. 1, 1996-97, pp. 1-9.
%D A107648 Ivan Niven, Herbert S. Zuckerman and Hugh L. Montgomery, An Introduction to the Theory Of Numbers, Fifth Edition, John Wiley and Sons, Inc., NY 1991, p. 141.
%H A107648 Patrick De Geest, World!Of Numbers, <a href="http://www.worldofnumbers.com/wing.htm#pwp181">Palindromic Wing Primes (PWP's)</a>
%H A107648 Makoto Kamada, <a href="https://stdkmd.net/nrr/1/11811.htm#prime">Prime numbers of the form 11...11811...11</a>
%H A107648 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F A107648 a(n) = (A077791(n)-1)/2.
%e A107648 7 is in the sequence because (10^15+63*10^7-1)/9=1(7).8.1(7)=111111181111111 is prime.
%e A107648 666 is in the sequence because (10^(2*666+1)+63*10^666-1)/9=1(666).8.1(666) is prime.
%t A107648 Do[If[PrimeQ[(10^(2n + 1) + 63*10^n - 1)/9], Print[n]], {n, 4000}]
%o A107648 (PARI) for(n=0,1e4,if(ispseudoprime(t=(10^(2*n+1)+63*10^n)\9),print1(t", "))) \\ _Charles R Greathouse IV_, Jul 15 2011
%Y A107648 Cf. A004023, A077775-A077798, A107123-A107127, A107648, A107649, A115073, A183174-A183187.
%K A107648 nonn,more,base
%O A107648 1,2
%A A107648 _Farideh Firoozbakht_, May 19 2005
%E A107648 Edited by _Ray Chandler_, Dec 28 2010
%E A107648 a(9) from _Robert Price_, Aug 03 2024