OFFSET
1,1
FORMULA
It appears that all even terms in the sequence are given by
a(2m) = (10^m)/2 + round(10^(m/2))
and all odd terms by
a(2m+1) = 10^m + floor(frac((sqrt(8*(100^m-1))-1)/2)/(1-sqrt(1/2))).
EXAMPLE
There exist 2-digit triangular numbers beginning with 1 (10, 15), 2 (21, 28), 3 (36), 4 (45), 5 (55), 6 (66), and 7 (78), but not 8, so a(2)=8.
There exist 5-digit triangular numbers beginning with every number from 1 through 99, and the first few 5-digit triangular numbers are 10011 (begins with 100), 10153 (begins with 101), 10296 (begins with 102), and 10440 (begins with 104), so a(5)=103.
CROSSREFS
KEYWORD
nonn,base
AUTHOR
Jon E. Schoenfield, May 03 2015
STATUS
approved