Well by observation n=1, k=3 is one solution. I don't think there are other solutions but I have no way of proving their aren't
See this web entry.
I have a great many different textbooks on set theory. In the vast majority of them use .
I haven't seen that the author had mentioned that the numbers are different from 0 . And also , in the some problems that i've seen if the numbers aren't null they are included in N* and in the problem the number are only from N .