[see also: investigation, study]
Research partially supported by NSF Grant No. 23456.
This research was initiated when the first author was visiting the University of Alberta in the summer of 2008; the financial support and kind hospitality are gratefully acknowledged.
A survey of the research on $f_n(x,y)$ up to 1970 (most of it dealing with the case $n=1$) was given in [3].
This is an interesting area for future research.
We close this paper by offering some questions and problems for further research.
This work continues research begun in [5].
In the present work we continue this line of research.
detailed $\langle$in-depth/extensive/pioneering/ground-breaking/thorough$\rangle$ research
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