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 A372207 #41 May 20 2024 16:51:12 %S A372207 23,1213,102101,10021001,1000810007,100010100011,10000241000023, %T A372207 1000004210000041,100000002100000003,10000000081000000009, %U A372207 1000000000810000000007,100000000002100000000001,10000000000021000000000001,1000000000001010000000000011,100000000000002100000000000003 %N A372207 a(n) is the smallest prime number that is obtained by concatenating 2 consecutive n-digit integers. %C A372207 It is possible to form primes by concatenating an integer with its successor (A030458), as well as by concatenating an integer with its predecessor (A052089). %H A372207 Michael S. Branicky, <a href="/A372207/b372207.txt">Table of n, a(n) for n = 1..500</a> %e A372207 a(3) = 102101, since when concatenating 3-digit numbers, the first three numbers obtained, in increasing order, are 100101, 101100, 101102, which are composite, while the next number in the list is 102101, which is prime. %o A372207 (Python) %o A372207 from sympy import isprime %o A372207 def a(n): %o A372207 if n == 1: return 23 %o A372207 lb, ub = 10**(n-1), 10**n %o A372207 for k in range(lb, ub, 2): %o A372207 base = k*ub %o A372207 for inc in [k-1, k+1]: %o A372207 if inc >= lb and isprime(t:=base+inc): %o A372207 return t %o A372207 print([a(n) for n in range(1, 16)]) # _Michael S. Branicky_, May 19 2024 %Y A372207 Cf. A030458, A052089. %K A372207 nonn,base %O A372207 1,1 %A A372207 _Gonzalo MartÃnez_, May 19 2024 %E A372207 a(12) and beyond from _Michael S. Branicky_, May 19 2024