“…There is a vast literature concerning this problem in the 20th century: Hadamard [22] in 1908, Courant and Hilbert [13] in the German edition of 1937, Polya and Szëgo [43] in 1951, Garabedian and Schiffer [19,20] in 1952-1953, Polya and Schiffer [42] in 1953, Schiffer [46] in 1954 and thereafter [2,18,3,27,39,[36][37][38]40,47,16,44,14,17,21]. For more recent advances, we cite the works [29,11,15,35,6,26,25,30,31,4,8,9,33,10,34,32]. We also mention three interesting works on generic properties of eigenvalues and eigenfunctions due to Uhlenbeck [48,49] and Pereira [41] which are closely related to the issue of this paper.…”