For the Paris problems, both banks of the Seine have seven bridges, an odd number. The two islands each have an even number of links. Thus, the left and right banks must be at the start and end of the route

Leonhard Euler, the outstanding Swiss mathematician, considered a problem known as The Seven Bridges of Königsberg. It involves a walk around the city(...)

Ukrainian servicemen at a checkpoint near Luhansk village, not far from Debaltseve, Donetsk area, Ukraine, on Tuesday. Lithuania has decided to restore compulsory military service for young men as tensions in Ukraine continue to worry the small Baltic nation. Photograph: Anastasia Vlasova/Reuters

Lithuania has decided to restore compulsory military service for young men as tensions in Ukraine continue to worry the small Baltic nation.After a me(...)

Swedish corvette HMS Visby patrols the Stockholm Archipelago, searching for what the military says is a foreign threat in the waters. Photograph: Marko Savala/Reuters

Sweden released a grainy photo of a mysterious vessel in Stockholm’s archipelago yesterday, as the military hunted for a foreign submarine or divers i(...)

President Barack Obama meets Nato secretary general Anders Fogh Rasmussen in Brussels on Wednesday. Photograph: Doug Mills/ The New York Times

US president Barack Obama’s four-day visit to Europe continued yesterday with a visit to the EU headquarters in Brussels, where he held talks w(...)

 A file photo dated from 2010 showing former head of Russian Yukos oil company Mikhail Khodorkovsky (centre) looks from a cage after the reading of his verdict in the Khamovnichesky district court in Moscow, Russia. Photograph: Maxim Shipenkov/EPA

Russian president Vladimir Putin says he will pardon jailed oil tycoon Mikhail Khodorkovsky, a surprise decision that will let his top foe and Russia’(...)

The Izvestia report followed a story in German newspaper Bild on Saturday that said secret satellite imagery showed Iskander-M missiles stationed near the Polish border. Photograph: Evgeny Stetsko/Getty Images

Russia has deployed Iskander missiles with a range of hundreds of kilometres in its Baltic Sea exclave of Kaliningrad, which borders Nato members Pola(...)

Students take part in a rally to support EU integration in central Kiev on Thursday. Photograph: Valentyn Ogirenko/Reuters

As political leaders gathered for dinner in the chilly Lithuanian capital of Vilnius yesterday evening, the joke circulating among officials was wheth(...)

Holidaymakers enjoying the sun were startled yesterday when the world’s largest hovercraft landed in their midst. The vessel was participating in military maneuvers.

Holidaymakers near the Russian port city of Kaliningrad were startled yesterday when a giant military hovercraft landed in their midst on a beach i(...)