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 A089922 #22 May 13 2025 08:26:07
%S A089922 3,73,123,817,5433
%N A089922 Largest odd number in the reverse concatenation of the first n consecutive odd numbers when that concatenation is prime.
%C A089922 These numbers are rare. For the case of the reverse concatenation of the consecutive numbers 1,2,3,... I have only found 828180...54321. Certainly, we can reduce the trials by noting that 2/3 of the numbers formed this way are multiples of 3 for the consecutive numbers and 1/3 of the odd consecutive numbers is a multiple of 3 but I do not think that will help much using PARI.
%C A089922 The number corresponding to 54335431...7531 has 10313 digits. The next term, if it exists is greater than 13000. - _Dmitry Kamenetsky_, Feb 21 2009
%e A089922 31 is the reverse concatenation of the consecutive odd numbers 1 and 3, and it is prime.
%e A089922 817815813...531 is the reverse concatenation of the odd numbers 1 to 817, and it is a probable prime.
%o A089922 (PARI) revprime2(n) = { y=1; forstep(x=3,n,2, y=concat(Str(x),Str(y)); z=eval(y); if(ispseudoprime(z),print(x, ",")) ) }
%Y A089922 Cf. original concatenation in A038395. - _Dmitry Kamenetsky_, Feb 21 2009
%K A089922 nonn,base,more,hard
%O A089922 1,1
%A A089922 _Cino Hilliard_, Jan 11 2004
%E A089922 Edited by _T. D. Noe_, Oct 30 2008
%E A089922 Added a new term 5433. This number is a probable prime with 20 iterations of Miller-Rabin test. - _Dmitry Kamenetsky_, Feb 21 2009