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 A103523 #29 Jul 05 2021 02:42:12
%S A103523 3103,7107,13113,31131,37137,67167,73173,79179,97197,127227,139239,
%T A103523 151251,157257,163263,181281,193293,211311,283383,331431,349449,
%U A103523 367467,379479,409509,421521,457557,463563,487587,499599,541641,547647,577677
%N A103523 Concatenations of pairs of primes that differ by 100.
%C A103523 Integers in this sequence can never be prime, as, starting from the second one, they are all multiples of 3.
%H A103523 Robert Israel, <a href="/A103523/b103523.txt">Table of n, a(n) for n = 1..9832</a>
%F A103523 List: concatenate(p, p+100) iff p and p+100 are primes.
%e A103523 9191019 is in this sequence because 919 is prime, 919+100 = 1019 is prime and 9191019 is the concatenation of those two primes differing by 100.
%p A103523 f:= proc(n) if isprime(n) and isprime(n+100) then 10^(1+ilog10(n+100))*n+n+100 fi end proc:
%p A103523 map(f, [3,seq(i,i=7..1000,6)]); # _Robert Israel_, Dec 07 2015
%t A103523 FromDigits[Join@@IntegerDigits/@{#,#+100}]&/@Select[Prime@Range@200,PrimeQ[#+100]&] (* _Giorgos Kalogeropoulos_, Jul 04 2021 *)
%o A103523 (Python)
%o A103523 from sympy import isprime, primerange as prange
%o A103523 def auptop(lim):
%o A103523 return [int(str(p)+str(p+100)) for p in prange(2, lim+1) if isprime(p+100)]
%o A103523 print(auptop(577)) # _Michael S. Branicky_, Jul 04 2021
%Y A103523 Cf. A000040, A001358, A023201, A100750, A103195, A103206, A104718, A104719.
%K A103523 base,easy,nonn
%O A103523 1,1
%A A103523 _Jonathan Vos Post_, Mar 21 2005