Viliam Geffert

{{short description|Slovak theoretical computer scientist}}

{{Infobox scientist

| name = Viliam Geffert

| birth_date = {{Birth year and age|1955}}

| birth_place =

| death_date =

| death_place =

| field = Automata theory, computational complexity

| work_institution = P. J. Šafárik University

| alma_mater = P. J. Šafárik University, Comenius University

| doctoral_advisor =

| known_for = state complexity, small-space complexity

| prizes =

| religion =

| footnotes =

}}

Viliam Geffert (born 1955) is a Slovak

theoretical computer scientist

known for his contributions

to the computational complexity theory in sublogarithmic space{{cite journal|last1=Geffert|first1=Viliam|title=Tally Versions of the Savitch and Immerman–Szelepcsényi Theorems for Sublogarithmic Space|journal=SIAM Journal on Computing|volume=22|issue=1|year=1993|pages=102–113|issn=0097-5397|doi=10.1137/0222009}}{{cite journal|last1=Geffert|first1=Viliam|last2=Mereghetti|first2=Carlo|last3=Pighizzini|first3=Giovanni|title=Sublogarithmic Bounds on Space and Reversals|journal=SIAM Journal on Computing|volume=28|issue=1|year=1998|pages=325–340|issn=0097-5397|doi=10.1137/S0097539796301306|hdl=2434/178756|s2cid=37853723 |hdl-access=free}}

and to the state complexity of two-way finite automata.{{cite journal|last1=Geffert|first1=Viliam|title=An alternating hierarchy for finite automata|journal=Theoretical Computer Science|volume=445|year=2012|pages=1–24|issn=0304-3975|doi=10.1016/j.tcs.2012.04.044|doi-access=free}}{{cite journal|last1=Geffert|first1=Viliam|last2=Mereghetti|first2=Carlo|last3=Pighizzini|first3=Giovanni|title=Complementing two-way finite automata|journal=Information and Computation|volume=205|issue=8|year=2007|pages=1173–1187|issn=0890-5401|doi=10.1016/j.ic.2007.01.008|doi-access=free}}

He has also developed new in-place sorting algorithms.{{cite journal|last1=Franceschini|first1=Gianni|last2=Geffert|first2=Viliam|title=An in-place sorting with O(nlog n) comparisons and O(n) moves|journal=Journal of the ACM|volume=52|issue=4|year=2005|pages=515–537|issn=0004-5411|doi=10.1145/1082036.1082037|arxiv=cs/0305005|s2cid=2148080}}{{cite book|last1=Geffert|first1=Viliam|last2=Gajdoš|first2=Jozef|title=SOFSEM 2011: Theory and Practice of Computer Science|chapter=In-Place Sorting|series=Lecture Notes in Computer Science|volume=6543|year=2011|pages=248–259|issn=0302-9743|doi=10.1007/978-3-642-18381-2_21|bibcode=2011LNCS.6543..248G|isbn=978-3-642-18380-5}}

He is a professor and the head of the computer science department at the P. J. Šafárik University in Košice.