The Oxford murders staged as series
In 1993 Oxford, an Argentine mathematics graduate student finds his landlady, Mrs. Eagleton, a former wartime codebreaker, dead. The logician Arthur Seldom has received a note announcing the first of a series marked by a circle, and further deaths follow, each announced by a new symbol (a fish shape, a triangle, a Pythagorean tetraktys). Seldom later reveals that the first death was a domestic killing inside the Eagleton household, that the later deaths were natural, and that he invented the 'series' to disguise it. The fiction then inspired a real killer who acted out the pattern, causing actual deaths.