%I
%S 1,120,3,741,153,6,171,780,190,10,111156,1128,131328,2145,15,3160,
%T 1176,4186,101926,152076,21,1225,102378,3240,5253,7260,1275,28,232903,
%U 113050,9316,1326,133386,2346,5356,36,1378,7381,133903,3403,2415,124251,1431
%N The smallest triangular number that contains the digits of n in its exact middle.
%C "Exact middle" means that the counts of digits of the triangular number to the left and to the right of the digits of n are the same.
%C Leading 0's are not allowed, e.g. 30135 is not considered to have the digits of 13 in its exact middle.  _Robert Israel_, Nov 22 2017
%C Essentially the same as A062690.  _Georg Fischer_, Oct 01 2018
%H Robert Israel, <a href="/A163598/b163598.txt">Table of n, a(n) for n = 1..10000</a>
%e a(22) = 1225 = 1//22//5 = A000217(49) is the smallest member of A000217 which displays 22 in the middle.  _R. J. Mathar_, Aug 11 2009
%p N:= 100: # to get a(1)..a(N)
%p Mid:= proc(t,d,i)
%p local s;
%p if d::odd then
%p s:= floor(t/10^((d+1)/2i)) mod 10^(2*i1);
%p if ilog10(s) < 2*i2 then s:= NULL fi
%p else s:= floor(t/10^(d/2i)) mod 10^(2*i);
%p if ilog10(s) < 2*i1 then s:= NULL fi
%p fi;
%p s
%p end proc:
%p V:= Vector(N):
%p count:= 0:
%p for k from 1 while count < N do
%p t:= k*(k+1)/2;
%p d:= ilog10(t)+1;
%p L:= [seq(Mid(t,d,i),i=1..(d+(d mod 2))/2)];
%p for x in L do
%p if x <= N and x > 0 and V[x] = 0 then
%p count:= count+1;
%p V[x]:= t;
%p fi
%p od
%p od:
%p convert(V,list); # _Robert Israel_, Nov 22 2017
%K nonn,base,look
%O 1,2
%A _Claudio Meller_, Aug 01 2009
%E a(20) corrected by _Robert Israel_, Nov 22 2017
