Looks like a random sequence...

Level 2

What is the next term? 1 , 2 , 3 , 5 , 6 , 8 , 9 , 11 , 14 , 15 , 18 , . . . 1, 2, 3, 5, 6, 8, 9, 11, 14, 15, 18, ...


The answer is 20.

Takeda Shigenori by Takeda Shigenori , 46, Malaysia

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3 solutions

Lewis Tough
Dec 30, 2013

The numbers in the sequence are all (odd primes-1) divided by 2. So, 18 would yield 37, which we know to be prime. The next prime is 41, take 1 to get 40 and then half to get 20.

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Nice one. Thanks!

Piyushkumar Palan - 7 years, 5 months ago
Ivan Koswara
Dec 30, 2013

oeis.org is a good resource. (Also one of the sequences returns a next term of 21 instead of 20, so the answer is not entirely valid. Besides, one can create an arbitrary sequence such that the first few terms match the given above and have the next term being completely arbitrary. But hey, just for fun, so I'll let it pass.)

2 Helpful 0 Interesting 0 Brilliant 0 Confused
Lee Gao
Dec 31, 2013

This is the sequence of the number of even numbers less than the k t h k^{th} prime

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