Between the nine planets of the solar system, a cosmic messaging system is introduced. Rockets fly along the following routes: Earth – Mercury, Pluto – Venus, Earth – Pluto, Pluto – Mercury, Mercury – Venus, Uranus – Neptune, Neptune – Saturn, Saturn – Jupiter, Jupiter – Mars and Mars – Uranus. Is it possible to get from Earth to Mars?
There are 9 cities in the country Number with the names 1, 2, 3, 4, 5, 6, 7, 8, 9. The traveller discovered that two cities are connected by an airline if and only if a two-digit number made up of the digit-names of these cities, is divisible by 3. Is it possible to get from city 1 to city 9?
In a state there are 100 cities, and from each of them there are 4 roads. How many roads are there in the state?
In the country Seven there are 15 cities, each of which is connected by roads with no less than seven other cities. Prove that from every city you can get to any other city (possibly passing through other cities).
There is a group of islands connected by bridges so that from each island one can get to any of the other islands. The tourist has bypassed all the islands, walking on each bridge exactly once. He visited the island of Three-isle three times. How many bridges are there on Three-isle if the tourist
a) did not start on it and did not finish on it?
b) started on it, but did not finish on it?
c) started on it and finished on it?
Prove that there is no graph with five vertices whose degrees are equal to 4, 4, 4, 4, 2.
Prove that a graph, in which every two vertices are connected by exactly one simple path, is a tree.
Prove that, in a tree, every two vertices are connected by exactly one simple path.
Eugenie, arriving from Big-island, said that there are several lakes connected by rivers. Three rivers flow from each lake, and four rivers flow into each lake. Prove that she is wrong.
Several Top Secret Objects are connected by an underground railway in such a way that each Object is directly connected to no more than three others and from each Object one can reach any other Object by going and by changing no more than once. What is the maximum number of Top Secret Objects?