diff --git a/permutation.h b/permutation.h
new file mode 100644
index 0000000..34c9934
--- /dev/null
+++ b/permutation.h
@@ -0,0 +1,9 @@
+template<typename T>
+const NRVec<T> inversepermutation(const NRVec<T> &p, const T offset=0)
+{
+int n=p.size();
+NRVec<T> q(n);
+if(!offset) for(int i=0; i<n; ++i) q[p[i]]=i;
+else for(int i=0; i<n; ++i) q[p[i]-offset]=i+offset;
+return q;
+}