Erik Palmgren An Effective Conservation Result for Nonstandard Arithmetic. 17-23 2000 46 Math. Log. Q. 1 http://dx.doi.org/10.1002/(SICI)1521-3870(200001)46:1<17::AID-MALQ17>3.0.CO;2-8 db/journals/mlq/mlq46.html#Palmgren00