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 A232126 #18 Nov 27 2013 05:20:36
%S A232126 53,5,3,2,2,3,2,2,19,103,409,1457011
%N A232126 First element of the chain of primes ending in A232125(n), prime which cannot be extended to another prime by appending a digit, as it is the case of the other elements of the chain.
%C A232126 See sequence A231426 for a variant using a similar concept "working forwards", i.e., the longest possible extension is looked for. See also A232127, A232128.
%F A232126 a(n) = A232125(n)/10^n.
%e A232126 a(0)=53 is the least prime that cannot be extended to another prime by appending some digit.
%e A232126 a(1)=5 is the least prime that can be extended ("once") to another prime, by appending the digit "3", such that the new prime cannot be extended further. (Indeed, 2 can be extended to 23 or 29, and 3 can be extended to 31 and 37, but all these allow at least one further extension to some prime, e.g., 233, 293, 311 and 373.)
%e A232126 a(3) = 2 is the first prime in the chain (2, 23, 239, 2393) where a digit is added 3 times to yield another prime, while adding any digit to the last term will give a composite. Here, 2393 is the least prime to occur in such a sequence of length 4=1+3.
%K A232126 nonn,base,more
%O A232126 0,1
%A A232126 _Michel Marcus_ and _M. F. Hasler_, Nov 19 2013