OFFSET
1,2
COMMENTS
"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.
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
Essentially the same as A062690. - Georg Fischer, Oct 01 2018
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
EXAMPLE
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
MAPLE
N:= 100: # to get a(1)..a(N)
Mid:= proc(t, d, i)
local s;
if d::odd then
s:= floor(t/10^((d+1)/2-i)) mod 10^(2*i-1);
if ilog10(s) < 2*i-2 then s:= NULL fi
else s:= floor(t/10^(d/2-i)) mod 10^(2*i);
if ilog10(s) < 2*i-1 then s:= NULL fi
fi;
s
end proc:
V:= Vector(N):
count:= 0:
for k from 1 while count < N do
t:= k*(k+1)/2;
d:= ilog10(t)+1;
L:= [seq(Mid(t, d, i), i=1..(d+(d mod 2))/2)];
for x in L do
if x <= N and x > 0 and V[x] = 0 then
count:= count+1;
V[x]:= t;
fi
od
od:
convert(V, list); # Robert Israel, Nov 22 2017
CROSSREFS
KEYWORD
AUTHOR
Claudio Meller, Aug 01 2009
EXTENSIONS
a(20) corrected by Robert Israel, Nov 22 2017
STATUS
approved