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 A280576 #10 Jan 09 2017 11:26:46
%S A280576 23,79,3137,3739,4751,6163,8387,8389,109111,113117,113123,151153,
%T A280576 151157,157163,167173,173177,181183,199207,199211,211213,211217,
%U A280576 211219,233239,241249,251257,257263,263267,263269,271273,271277,277279,283289,317321,317323,317327
%N A280576 Primes formed from the concatenation of previousprime(n) and n.
%H A280576 K. D. Bajpai, <a href="/A280576/b280576.txt">Table of n, a(n) for n = 1..7768</a>
%e A280576 79 is in the sequence because it is a prime formed from the concatenation of 7 and 9, where 7 is the largest prime < 9.
%e A280576 8387 is in the sequence because it is a prime formed from the concatenation of 83 and 87, where 83 is the largest prime < 87.
%t A280576 Select[Table[FromDigits[Join[IntegerDigits[Prime[PrimePi[n - 1]]], IntegerDigits[n]]], {n,3,1000}], PrimeQ]
%o A280576 (Magma) [p : n in[3..200] | IsPrime (p) where p is Seqint (Intseq (n) cat Intseq (PreviousPrime (n)))];
%o A280576 (PARI) terms(n) = my(i=0, x=3); while(1, my(cc=eval(Str(precprime(x-1), x))); if(ispseudoprime(cc), print1(cc, ", "); i++); if(i==n, break); x++)
%o A280576 /* Print initial 40 terms as follows: */
%o A280576 terms(40) \\ _Felix Fröhlich_, Jan 05 2017
%Y A280576 Cf. A000040, A084667, A084669, A151799, A151800, A280357, A280388.
%K A280576 nonn,base
%O A280576 1,1
%A A280576 _K. D. Bajpai_, Jan 05 2017