We define recursively the Ulam numbers by setting u1 = 1, u2 = 2, and for each subsequent integer n, we set n equal to the next Ulam number if it can be written uniquely as the sum of two different Ulam numbers; e.g.: u3 = 3, u4 = 4, u5 = 6, etc.

Prove that there are infinitely many Ulam numbers.

Now a D4 BONUS.
3 (=1+2). Find another Ulam number is that is the sum of two consecutive Ulam numbers.

How about a 3rd one?

The Ulam numbers are OEIS A002858.  In the comments: "3 (=1+2) and 131 (=62+69) are the only two Ulam numbers in the first 28 billion Ulam numbers that are the sum of two consecutive Ulam numbers."

Posted by Brian Smith on 2017-06-04 09:42:56