The sequence is a subset of the prime numbers. Each subsequent number is N prime numbers apart. The first number, 2, is the first prime. The next number 5, is (two) prime numbers from the number 2. The next number 13 is (three) prime numbers from 5. The next number 31 is (five) primes away from 13. 61 is (seven) primes from 31, and 109 is (eleven) primes from 61. Does the sequence of the numbers in parenthesis look familiar? Its the primes! So you need the prime that is thirteen primes away from 109, and then the one that is seventeen primes away from the new number. 181 and 277.
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