Признанный специалист по роду Лапчатка.
Научные работы по ботанической номенклатуре
- Sojak, J., 1986. Notes on Potentilla. I. Hybridogenous species derived from intersectional hybrids of sect. Niveae X sect. Multifideae. Botanische Jahrb?cher f?r Systematic 106: 145—210
- Sojak, J. 1979: Fragmenta phylotaxonomica etnomenclatorica 1. Casopis Narodniho Muse a (Prague) Oddil Priroddovedny 148: 193—209.
- Soj?k, J. 2004. Potentilla L. (Rosaceae) and related genera in the former USSR (identification key, checklist and figures). Notes on Potentilla XVI. — Bot. Jahrb. Syst. 125: 253—340.
- Soj?k, J. 2006. New infraspecific nomenclatural combinations in twelve American species of Drymocallis and Potentilla (Rosaceae) (Notes on Potentilla XVII.). — Thaiszia — J. Bot. 16: 47-50
- Sojak, J., 1989. Notes on Potentilla (Rosaceae) VIII. P. Mivea L. agg. Candollea 44: 741-62
- Sojak, J. 1972 : Dopinky k nomenklatufe nekterych rod e (Phanerogamae). Casopis Narodniho Muse a (Prague) Oddil Priroddovedny 141: 61-63.
- Sojak, J., 1985. Some new northern hybrids in Potentilla L. Preslia 57: 263—266
