107
double difference each time
2 + 7 = 9
9 + 7 + 7 = 23
23 + 14 + 14 = 51
51 + 28 + 28 = 107
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158 One possible sequence is t(n) = 0.5*(19n2 - 7n + 40) for n = 1, 2, 3, ...
51 is not a prime number: 3x17=51
51 is composite number
51. Just add two to an odd number to work out the next ;)
23 is the prime number.