*1 ejemplo de frase nominal con post-modificadores
*2 ejemplos de oraciones con "there be"
*1 ejemplo de frase verbal en presente perfecto, presente continuo, presente simple, pasado simple y futuro simple
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There are hundreds of unsolved problems in number theory, of which have already been mentioned. This concluding section lists examples of problems that ar easy to state but which remain unsolved. Prime numbers and divisibility Are there infinitely many even perfect numbers? Are there any odd perfect numbers? Is there an even number greater that 2 that is not the sum of two primes? Is there an even number greater that 2 that is not the difference of two primes? The intellectual challange of unsolved problems in number theory has kept the subject alive and active since the time of the greeks who asked all these questions long before modern mathematicians. The mathematics departments of all the most outstanding universities of the world are intending to find answers to these problems. But because new problems arise more rapidly that the old ones are solved, number theory will continue to thrive and to stimulate other branches of mathematics for centuries to come.