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 A101396 #28 Jul 02 2025 15:53:17
%S A101396 1,17,19,29,43,119,173,949,1609,5579,19679,34147,43493,97799
%N A101396 Numbers k such that 4*10^k - 9 is prime.
%C A101396 a(15) > 10^5. - _Robert Price_, Mar 17 2015
%C A101396 All terms are odd, since 4 * 10^(2*k) - 9 = (2 * 10^k - 3)*(2 * 10^k + 3). - _Robert Israel_, Mar 17 2015
%H A101396 Makoto Kamada, <a href="https://stdkmd.net/nrr/3/39991.htm#prime">Prime numbers of the form 399...991</a>.
%H A101396 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F A101396 a(n) = A101848(n) + 1.
%e A101396 n = 1, 17, 19 are members since 31, 399999999999999991 and 39999999999999999991 are primes.
%p A101396 select(n -> n::odd and isprime(4*10^n-9), [$1..10000]); # _Robert Israel_, Mar 17 2015
%t A101396 Do[ If[ PrimeQ[4*10^n - 9], Print[n]], {n, 0, 10000}]
%o A101396 (Magma) [n: n in [1..300] |IsPrime(4*10^n - 9)]; // _Vincenzo Librandi_, Mar 18 2015
%o A101396 (PARI) is(n)=ispseudoprime(4*10^n-9) \\ _Charles R Greathouse IV_, Jun 12 2017
%Y A101396 Cf. A095714, A101848.
%K A101396 more,nonn
%O A101396 1,2
%A A101396 Julien Peter Benney (jpbenney(AT)ftml.net), Jan 15 2005
%E A101396 a(11) from Kamada link by Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008
%E A101396 a(12)-a(14) from Kamada data by _Robert Price_, Mar 17 2015