diff --git a/quad10/fenomens/lab/p4/pbc.f90 b/quad10/fenomens/lab/p4/pbc.f90
new file mode 100644
index 0000000..09cfff3
--- /dev/null
+++ b/quad10/fenomens/lab/p4/pbc.f90
@@ -0,0 +1,11 @@
+integer*4 function PBC(N, L)
+  integer*4, intent(in) :: N, L
+  if (N == 0) then
+    PBC = L
+  else if (N == L + 1) then
+    PBC = 1
+  else
+    PBC = N
+  endif
+  return
+endfunction