On Sat, 3 Aug 2002, Wouter Meeussen wrote: > ID Number: A007595 (Formerly M2681) > Sequence: 1,1,3,7,22,66,217,715,2438,8398,29414 > (C_n + C_(n-1)/2 )/2, C = Catalan numbers (A000108). > > can't work 'cause C_(2^n-1) is odd, (usually). ... The following is known: C_n is odd if and only if n=2^k-1, k=1,2,3,... . Emeric