ABC2NUM(S) returns the number corresponding to the spreadsheet column represented by the reference string, S. The input string, S, may contain the ascii letters A-Z, in the format commonly employed by spreadsheet programs such as Calc and Microsoft Excel. Other characters, including lower case letters, a-z, are ignored.
NUM2ABC(D) returns the A1-style string label corresponding to the appropriate spreadsheet column represented by the specified integer, D. The specified integer must be greater than or equal to one.
 Abraham, R., Burnett, M. and Erwig, M., "Spreadsheet Programming" Encyclopedia of Computer Science and Engineering, (ed. B.J. Wah), 2804-2810, 2009
Please note: There are many interesting submissions on FEX which provide similar results (see below).
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