add reference

Author: lua-vr

docs/references.bib

@@ -3602,6 +3602,18 @@ @Article{         kratschmer_urusov2023
   url           = {https://doi.org/10.1007/s10959-022-01207-8}
 }
 
+@Book{            krengelErgodicTheorems2011,
+  title         = {Ergodic {{Theorems}}},
+  author        = {Krengel, Ulrich},
+  year          = 2011,
+  month         = mar,
+  publisher     = {De Gruyter},
+  doi           = {10.1515/9783110844641},
+  urldate       = {2026-06-03},
+  isbn          = {978-3-11-084464-1},
+  langid        = {english}
+}
+
 @Book{            kung_rota_yan2009,
   author        = {Kung, Joseph P. S. and Rota, Gian-Carlo and Yan, Catherine
                   H.},