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A sequence is defined as shown for n = 3, 4, 5, ... What will be the first number in this sequence that is NOT prime? 

 

a_1 = 11

a_2 = 2

a_3 = a_2 + a_1

a_4 = a_3 + a_2 + 2

a_5 = a_4 + a_3 + 4

...

a_{n + 1} = a_n + a_{n - 1} + 2(n - 2)

 Jul 12, 2022
 #1
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a3  =  13

a4  =  17

a5  =  34   not prime

 

cool cool cool

 Jul 13, 2022

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