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.
a(12) = 2112, a(1235) = 53211235
12 is a member as 2112 is divisible by 12 and 13 is not as 3113 is not divisible by 13.
filter:= proc(n) local L,m,i,v;
L:= convert(n,base,10);
m:= nops(L);
v:= add(10^(2*m-i)*L[i],i=1..m);
evalb(v mod n = 0);
end proc:
select(filter, [$1..10000]); # Robert Israel, Jul 26 2015
Select[ Range[ 250 ],
Divisible[
FromDigits[
Flatten[ { Reverse[ IntegerDigits[ # ] ] ,
IntegerDigits[ # ] } ] ], # ] & ]
(* Kevin Southwick, Jul 25 2015 *)
Select[Range[250],Divisible[IntegerReverse[#]*10^IntegerLength[#]+#,#]&] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Mar 28 2020 *)
f:= proc(n) local L,m,i,v;
L:= convert(n,base,10);
m:= nops(L);
v:= add(10^(2*m-i)*L[i],i=1..m)/n+1;
if v::integer then v else NULL fi
end proc:
map(f, [$1..1000]); # Robert Israel, Jul 26 2015
A083970 = Select[ Range[ 250 ], Divisible[ FromDigits[ Flatten[ { Reverse[ IntegerDigits[ # ] ], IntegerDigits[ # ] } ] ], # ] & ]; Table[ FromDigits[ Flatten[ { Reverse[ IntegerDigits[ tmp ] ], IntegerDigits[ tmp ] } ] ] / tmp, {tmp, A083970} ] (* Kevin Southwick, Jul 26 2015 *)
a(12) = 2112, a(1235) = 53211235
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