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 A217559 #9 Dec 23 2024 14:53:43
%S A217559 1,2,4,3,6,5,8,7,9,23,10,29,22,11,24,31,25,13,12,17,14,37,15,19,26,53,
%T A217559 16,59,28,71,30,73,18,79,20,211,21,223,48,227,49,229,27,41,32,43,44,
%U A217559 233,45,47,46,239,33,61,34,241,35,67,60,251,36,257,62,83,63,89,38,263,64,269,39,271,50,277
%N A217559 Terms as well as ending/starting digits have alternating primality; this is the lexicographically earliest injective sequence with this property.
%C A217559 Exactly every other term is prime; moreover the ending digit of a(n) and the initial digit of a(n+1) are never both prime or both composite.
%H A217559 E. Angelini, <a href="https://web.archive.org/web/*/http://list.seqfan.eu/oldermail/seqfan/2012-October/010253.html">Non-primes/primes: integers and digits alternate</a>, SeqFan mailing list, Oct 06 2012
%o A217559 (PARI) A217559(n,show_all=0,a=1,u)={for( i=2, n, u+=1<<a; show_all & print1(a","); for(t=1,9e9, bittest(u,t) & next; isprime(t)==bittest(i,0) & next; isprime(a%10) == isprime(t\10^(#Str(t)-1)) & (t+=10^(#Str(t)-1)-1) & next; a=t; break )); a}
%Y A217559 This is a simplified variant of A217560.
%Y A217559 See also A217555, A217556, where "primality" is replaced by "parity".
%K A217559 nonn,base
%O A217559 1,2
%A A217559 _Eric Angelini_ and _M. F. Hasler_, Oct 06 2012