Each successive number is the sum of the previous number times 2, plus 1.
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Add 3 each time
3 is the answer
The pattern will be +2, +3, +4, +4
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One possible rule is: Un = (243n5 - 4210n4 + 27225n3 - 60830n2 + 106452n - 45960)/120 for n = 1, 2, 3, etc