std::vector<std::vector<double> > a(n,n+1);

$ c++ test.cc 
/usr/include/c++/4.3/bits/stl_vector.h: In member function ‘void std::vector<_Tp, _Alloc>::_M_initialize_dispatch(_Integer, _Integer, std::__true_type) [with _Integer = int, _Tp = std::vector<double, std::allocator<double> >, _Alloc = std::allocator<std::vector<double, std::allocator<double> > >]’:
/usr/include/c++/4.3/bits/stl_vector.h:290:   instantiated from ‘std::vector<_Tp, _Alloc>::vector(_InputIterator, _InputIterator, const _Alloc&) [with _InputIterator = int, _Tp = std::vector<double, std::allocator<double> >, _Alloc = std::allocator<std::vector<double, std::allocator<double> > >]’
test.cc:123:   instantiated from here
/usr/include/c++/4.3/bits/stl_vector.h:932: ошибка: нет подходящей функции для вызова ‘std::vector<std::vector<double, std::allocator<double> >, std::allocator<std::vector<double, std::allocator<double> > > >::_M_fill_initialize(size_t, int&)’
/usr/include/c++/4.3/bits/stl_vector.h:974: замечание: претенденты: void std::vector<_Tp, _Alloc>::_M_fill_initialize(size_t, const _Tp&) [with _Tp = std::vector<double, std::allocator<double> >, _Alloc = std::allocator<std::vector<double, std::allocator<double> > >]