▁theory . ▁ ▁After ▁comple ting ▁his ▁master ' s ▁degree ▁at ▁Le iden ▁University ▁from ▁ 1 9 1 8 – 1 9 2 2 ▁he ▁studied ▁at ▁the ▁University ▁of ▁C open hagen ▁with ▁Har ald ▁Bo hr ▁and ▁the ▁University ▁of ▁Oxford ▁with ▁G . ▁H . ▁Hard y . ▁In ▁ 1 9 2 4 ▁he ▁received ▁his ▁Ph . D . ▁in ▁Le iden ▁under ▁super vision ▁of ▁J . ▁C . ▁K lu y ver . ▁From ▁ 1 9 2 6 ▁to ▁ 1 9 2 8 ▁he ▁studied ▁at ▁the ▁univers ities ▁of ▁Gött ingen ▁and ▁Hamburg , ▁and ▁he ▁was ▁assistant ▁at ▁the ▁University ▁of ▁Mün ster ▁from ▁ 1 9 2 8 - 1 9 3 0 . ▁K lo ost erman ▁was ▁appointed ▁le ctor ▁( associ ate ▁professor ship ) ▁at ▁ ▁Le iden ▁University ▁in ▁ 1 9 3 0 ▁and ▁full ▁professor ▁in ▁ 1 9 4 7 . ▁In ▁ 1 9 5 0 ▁he ▁was ▁elected ▁a ▁member ▁of ▁the ▁Royal ▁Netherlands ▁Academy ▁of ▁Arts ▁and ▁Sciences . ▁ ▁References ▁ ▁External ▁links ▁▁▁ ▁Category : 1 9 0 0 ▁birth s ▁Category : 1 9 6 8 ▁death s ▁Category : 2 0 th - century ▁Dutch ▁mathemat icians ▁Category : Le iden ▁University ▁al umn i ▁Category : Le iden ▁University ▁fac ulty ▁Category : M embers ▁of ▁the ▁Royal ▁Netherlands ▁Academy ▁of ▁Arts ▁and ▁Sciences ▁Category : Number ▁the or ists ▁Category : Pe