: ▁Tour ism ▁in ▁the ▁XX I ▁century , ▁Sof ia , ▁ 2 0 0 2 , ▁p .   4 0 0 – 4 0 6 . ▁▁ ▁Transport ▁services ▁to ▁the ▁population ▁in ▁Bulg aria . ▁– ▁In : ▁Iz v . ▁Un iv . ▁Press , ▁V arna , ▁ 2 0 0 2 , ▁N : ▁ 1 , ▁p .   4 1 – 5 1 . ▁ ▁( in ▁collaboration ▁with ▁Mar in ▁Na iden ov ▁N esh kov ) ▁Tour ism ▁science ▁at ▁the ▁end ▁of ▁XX ▁century . ▁– ▁In : ▁Econom ic ▁and ▁social ▁development , ▁V arna , ▁UE , ▁ 2 0 0 2 , ▁p .   3 5 4 – 3 6 0 . ▁ ▁Inter - m unicip al ▁bus ▁lines ▁in ▁Bulg aria ▁– ▁functional ▁and ▁territorial ▁condition ing . ▁– ▁In : ▁Iz v . ▁Un iv . ▁Press , ▁V arna , ▁ 1 9 9 7 , ▁N : ▁ 2 , ▁p .   4 3 – 5 7 . ▁ ▁Transport ation ▁z oning ▁of ▁Bulg aria . ▁– ▁In : ▁Year book ▁Un iv . ▁E con . ▁V arna , ▁ 6 7 , ▁ 1 9 9 5 , ▁p .   3 – 5 3 . ▁ ▁Walter ▁Christ aller ▁and ▁his ▁central ▁place ▁theory ▁– ▁In : ▁Problem s ▁of ▁Geography , ▁ 1 9 9 4 , ▁N : ▁ 2 , ▁p .   1