How do I find subsequent work done on a paper? This question consists of a general question and a specific question. My general question is simple. Suppose I read a paper I like and I want to find out more about the topic. How can I do this ? The natural place to start is to go through the references at the end of the paper. However, there is a problem with this. References are only provided in one direction in time, namely the past. If the paper I'm reding was written in 1960, there's a good chance that extensive work has been done on the topic. How do I find references to how the topic has progressed after the paper has been published ?
Now, for my specific question, which is a special case of my general question. 
Courtesy of the wonderful Canadian Mathematical Society which allows free access to their back issues, I discovered a paper written in 1959 in the Canadian Mathematical Bulletin. The paper answers the question about how the set of non negative integers can be partitioned into two sets such that sums of pairs of distinct integers in both classes will be the same.
Here's a link to the paper. 
http://cms.math.ca/openaccess/cmb/v2/cmb1959v02.0085-0089.pdf
This paper is special to me, because it's the first research paper I read where I understood everything ! So, I was wondering if you guys could recommend some sources to me which either carry the problem forward, or which are written at the same level as this paper. 
 A: Concerning the specific question of the 1959 paper, here's a reviewer's note on MathSciNet, written by Ernst Straus: 
"Reviewer's note: While the method of proof, by a clever use of generating functions, uses the fact that one is dealing with integers, the result is clearly valid for arbitrary infinite cyclic semi-groups and, by an obvious extension, for direct products of such semi-groups. This includes the case of the natural numbers under multiplication which is treated separately in the paper."
The paper is referred to in reviews of two other papers: 
MR0399468 (53 #3312) 
Straus, E. G.
Real analytic functions as ratios of absolutely monotonic functions. Spline functions and approximation theory (Proc. Sympos., Univ. Alberta, Edmonton, Alta., 1972), pp. 359–370. Internat. Ser. Numer. Math., Vol. 21, Birkhäuser, Basel, 1973.  
MR0492264 (58 #11408) 
Goldstein, R.
On meromorphic solutions of certain functional equations. 
Aequationes Math. 18 (1978), no. 1-2, 112–157. 
Also, this paper cites Lambek-Moser in its references: 
MR2014728 (2004m:68172) 
Allouche, Jean-Paul; Shallit, Jeffrey
The ring of k-regular sequences. II.  Words. 
Theoret. Comput. Sci. 307 (2003), no. 1, 3–29. 
A: I don't know much about number theory, so i will only give a possible answer to the first, more general question (which probably deserves a thread on its own).
One possibility I know of is to use paperscape. Paperscape is a "landscape representation" of arXiv (basically a huge graph), clustering together related papers. There is also the very nice function allowing you to select a paper and "export it" to my.paperscape, where you can see all other papers cited and citing it.
Of course, the disadvantage is that you only get to see paper that are on arXiv.
