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.
Table[FromDigits[Flatten[IntegerDigits/@{n,n+2}]],{n,50}] (* Harvey P. Dale, Dec 18 2013 *)
Table[FromDigits[Join[IntegerDigits[n],IntegerDigits[n+3]]],{n,50}] (* Harvey P. Dale, Jul 11 2011 *)
#[[1]]*10^IntegerLength[#[[2]]]+#[[2]]&/@Table[{n,n+3},{n,50}] (* Harvey P. Dale, May 21 2018 *)
a[n_]:=n*10^Floor[Log10[n+9]+1]+n+9; Array[a,46] (* Stefano Spezia, Sep 04 2023 *)
[Seqint(Intseq(n) cat Intseq(n) cat Intseq(n)): n in [1..46]]; // Vincenzo Librandi, Feb 21 2014
Table[FromDigits[Flatten[IntegerDigits/@PadRight[{},3,n]]],{n,40}] (* Harvey P. Dale, Aug 10 2019 *)
10 cls 30 for I=1 to 100 40 A=str(I) 50 C=A+A+A 60 B=val(cutspc(C)) 80 print B 90 next 100 end
Concatenation 2 and 2 is 22; 3 and 3 is 33; 5 and 5 is 55; etc.
[Seqint(Intseq(p) cat Intseq(p)): p in PrimesUpTo(200)]; // Vincenzo Librandi, Mar 14 2013
dp[n_] := Module[{idn = IntegerDigits[n]}, FromDigits[Join[idn, idn]]]; dp /@ Prime[Range[40]] (* Harvey P. Dale, Jun 02 2011 *)
a(n) = { my(p=Str(prime(n))); eval(concat(p,p)); } /* Joerg Arndt, Mar 14 2013 */
a(1)=55 because 5^1 concatenated with 5^1 is 55.
[Seqint(Intseq(5^n) cat Intseq(5^n)): n in [0..20]];
Table[FromDigits[Join[IntegerDigits[5^n],IntegerDigits[5^n]]],{n,0,20}]
[Seqint(Intseq(n^2) cat Intseq(n^2)): n in [1..40]];
seq(n^2*(1+10^(1+ilog10(n^2))),n=1..100); # Robert Israel, Jan 02 2015
Table[FromDigits[Join[IntegerDigits[n^2], IntegerDigits[n^2]]], {n, 40}]
vector(100,n,eval(concat(Str(n^2),Str(n^2)))) \\ Derek Orr, Jan 02 2015