Double-negation topology ($J(A)=\{R:\forall f:B\to A,f^{\ast }R\ne \varnothing \}$)

The dense sieves form the double-negation Grothendieck topology. Its sheaf topos is a Boolean topos: the union of a subobject with its pseudocomplement is dense and thus becomes the whole object after sheafification. This need not be the atomic topology, which requires every nonempty sieve to be dense.

Consider a site indexed by natural-number objects with arrows only downwards, the cancellation condition $fh=gk\Rightarrow f=g$ for equally typed $f,g$, and the separation condition that arrows $f:m\to n$, $g:m\to n+1$ admit $h,k:m+1\to m$ with $fh=fk$ but $gh\ne gk$. Its dense-sieve sheaf topos is a Boolean topos and a two-valued topos. The representables are separated. No nonempty partial map $y_n\to y_{n+1}$ exists, by the separation condition and naturality. The associated sheaves $A_n$ consequently admit no maps $y_n\to A_{n+1}$. Each has nonzero support and maps epimorphically to one, but their product has an empty value at every stage $m$, because its $A_{m+1}$ factor does.