Ulam Sequence
The Ulam sequence ![<span style=]()
{a_i}=(u,v)" src="https://mathworld.wolfram.com/images/equations/UlamSequence/Inline1.gif" style="height:15px; width:67px" /> is defined by 
, 
, with the general term 
for 
given by the least integer expressible uniquely as the sum of two distinct earlier terms. The numbers so produced are sometimes called u-numbers or Ulam numbers.
The first few numbers in the (1, 2)-Ulam sequence are 1, 2, 3, 4, 6, 8, 11, 13, 16, ... (OEIS A002858). Here, the first term after the initial (1, 2) is obviously 3 since 
. The next term is 
. (We don't have to worry about 
since it is a sum of a single term instead of distinct terms.) 5 is not a member of the sequence since it is representable in two ways, 
, but 
is a member.
Proceeding in the manner, we can generate Ulam sequences for any 
, examples of which are given in the table below.
 |
Sloane |
sequence |
| (1, 2) |
A002858 |
1, 2, 3, 4, 6, 8, 11, 13, 16, 18, ... |
| (1, 3) |
A002859 |
1, 3, 4, 5, 6, 8, 10, 12, 17, 21, ... |
| (1, 4) |
A003666 |
1, 4, 5, 6, 7, 8, 10, 16, 18, 19, ... |
| (1, 5) |
A003667 |
1, 5, 6, 7, 8, 9, 10, 12, 20, 22, ... |
| (2, 3) |
A001857 |
2, 3, 5, 7, 8, 9, 13, 14, 18, 19, ... |
| (2, 4) |
A048951 |
2, 4, 6, 8, 12, 16, 22, 26, 32, 36, ... |
| (2, 5) |
A007300 |
2, 5, 7, 9, 11, 12, 13, 15, 19, 23, ... |
Schmerl and Spiegel (1994) proved that Ulam sequences 
for odd 
have exactly two even terms. Ulam sequences with only finitely many even terms eventually must have periodic successive differences (Finch 1991, 1992abc). Cassaigne and Finch (1995) proved that the Ulam sequences 
for 
(mod 4) have exactly three even terms.
The Ulam sequence can be generalized by the s-additive sequence.
REFERENCES:
Cassaigne, J. and Finch, S. "A Class of 1-Additive Sequences and Quadratic Recurrences." Exper. Math 4, 49-60, 1995.
Finch, S. "Conjectures About 1-Additive Sequences." Fib. Quart. 29, 209-214, 1991.
Finch, S. "Are 0-Additive Sequences Always Regular?" Amer. Math. Monthly 99, 671-673, 1992a.
Finch, S. "On the Regularity of Certain 1-Additive Sequences." J. Combin. Th. Ser. A 60, 123-130, 1992b.
Finch, S. "Patterns in 1-Additive Sequences." Exper. Math. 1, 57-63, 1992c.
Finch, S. R. "Stolarsky-Harborth Constant." §2.16 in Mathematical Constants. Cambridge, England: Cambridge University Press, pp. 145-151, 2003.
Guy, R. K. "A Quarter Century of Monthly Unsolved Problems, 1969-1993." Amer. Math. Monthly 100, 945-949, 1993.
Guy, R. K. "Ulam Numbers." §C4 in Unsolved Problems in Number Theory, 2nd ed. New York: Springer-Verlag, pp. 109-110, 1994.
Guy, R. K. and Nowakowski, R. J. "Monthly Unsolved Problems, 1969-1995." Amer. Math. Monthly 102, 921-926, 1995.
Recaman, B. "Questions on a Sequence of Ulam." Amer. Math. Monthly 80, 919-920, 1973.
Schmerl, J. and Spiegel, E. "The Regularity of Some 1-Additive Sequences." J. Combin. Theory Ser. A 66, 172-175, 1994.
Sloane, N. J. A. Sequences A001857/M0634, A002858/M0557, A002859/M2303, A003666/M3237, A003667/M3746, and A007300/M1328 in "The On-Line Encyclopedia of Integer Sequences."
Wolfram, S. A New Kind of Science. Champaign, IL: Wolfram Media, p. 908, 2002.
