When a dollar makes a BWT

The Burrows-Wheeler-Transform (BWT) is a reversible string transformation which plays central role in text compression and fundamental many modern bioinformatics applications. BWT permutation of the characters, general better compressible allows to answer several different query types more efficiently than original string. It easy see that not every image, exact characterizations images are known. We investigate related combinatorial question. In applications, sentinel character dollar added mark end string, thus ending with contains exactly one dollar-character. Given w, we ask positions, if any, dollar-character can be inserted turn w into image word dollar. show this depends only on standard present O(n log n)-time algorithm for identifying all such improving naive quadratic time algorithm. also give characterization positions develop bounds their number value. This an extended version [Giuliani et al. ICTCS 2019].