Perhaps you should try some spelling variations:
"problema 1.1 agente viajero" » "problema 11 agente viajero", "problema 1.2 agente viajero", "problema 19 agente viajero", "problema 1.1.2 agente viajero", "problema 1.2.1 agente viajero"
"problema 18 agente viajero" » "problema 1.2 agente viajero", "problema el agente viajero", "problema de agente viajero"
"problema 18 agente viajeros" » "problema 1.2 agente viajeros", "problema el agente viajeros", "problema de agente viajeros"

Try widening your search to All Fields.

You may be able to get more results by adjusting your search query.

• Performing a fuzzy search may retrieve terms with similar spellings: (((("problema 18 agente viajero") OR ("problema 1.1 agente viajero"))) or ("problema 18 agente viajeros"))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: (((("problema 18 agente viajero") OR ("problema 1.1 agente viajero"))) OR ("problema 18 agente viajeros")).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((((\"problema 18 agente viajero\") OR (\"problema 1.1 agente viajero\"))) or (\"problema 18 agente viajeros\"))".
• Removing quotes may allow a broader search: (((( problema 18 agente viajero ) OR ( problema 1.1 agente viajero ))) or ( problema 18 agente viajeros )).
• Adding a wildcard symbol may retrieve word variants: (((("problema 18 agente viajero") OR ("problema 1.1 agente viajero"))) or ("problema 18 agente viajeros"))*.