1,1 dimethylcyclopropene : molecular formula C5H8 ; Double bond equivalent(D.B.E) = 2 (one ring + one double bond)
Pentene : molecular formula : C5H10 ; D.B.E = 1 (for double bond only)
So, 1,1 dimethylcyclopropene is not a possible isomer of Pentene , Since both having different molecular formula