Appell is the same person: it stands for Monsieur Paul Appell.Paul Appell (27 September 1855 in Strasbourg – 24 October 1930 in Paris), also known as Paul Émile Appel, was a French mathematician and Rector of the University of Paris.
The concept of Appell polynomials is named after him, as is rue Paul Appell in the 14th arrondissement of Paris and the minor planet 988 Appella.