OFFSET
1,2
COMMENTS
From Robert Israel, Dec 30 2018: (Start)
All terms end in 0, 1, 3, 5, 6 or 8.
Triangular numbers that cannot be obtained as concatenation of a(n-1) and a(n) include 1, 3, 6, 10, 21, 28, 45, 78, 91. (End)
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
EXAMPLE
a(2) = 5 and a(3) = 28 and 528 is a triangular number.
MAPLE
Seen:= {}:
A[1]:= 1:
maxb:= 1:
for n from 2 to 100 do
found:= false;
for d from 1 while not found do
r:= A[n-1]*10^d;
x0:= r + 10^(d-1);
for m from ceil((sqrt(1+8*x0)-1)/2) do
x:= m*(m+1)/2;
if x >= r + 10^d then break fi;
if not member(x, Seen) then
A[n]:= x - r;
Seen:= Seen union {x};
found:= true;
break
fi
od od od:
seq(A[n], n=1..100); # Robert Israel, Dec 31 2018
CROSSREFS
KEYWORD
base,nonn
AUTHOR
Amarnath Murthy, Apr 10 2003
EXTENSIONS
More terms from Antonio G. Astudillo (afg_astudillo(AT)lycos.com), Apr 20 2003
Edited by Robert Israel, Dec 30 2018
STATUS
approved