You're at a party. Any two (different) people at the party have exactly one friend in common (the friend is also at the party). Prove that there is at least one person at the party who is a friend of everyone else. Assume that the friendship relation is symmetric and not reflexive.

Party Solution

PHP Warning

lib/DbaDatabase.php:154: Warning: dba_optimize() [<a href='function.dba-optimize'>function.dba-optimize</a>]: You cannot perform a modification to a database without proper access

lib/DbaDatabase.php:134: Warning: dba_replace() [<a href='function.dba-replace'>function.dba-replace</a>]: You cannot perform a modification to a database without proper access