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an=an−1+gcd(n,an−1) \large a_n = a_{n-1} + \gcd(n,a_{n-1}) an=an−1+gcd(n,an−1)
Consider the recurrence relation above with for n≥2n\geq2n≥2 with a1=7a_1 = 7a1=7. And define bn=an+1−anb_n= a_{n+1} - a_n bn=an+1−an, find the number of composite numbers bnb_nbn for n≤109n\leq10^9 n≤109.
For the sake of this question, take 1 as neither prime nor composite.
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