Solution:
First, we characterize all ordered pairs of general (not necessarily odd) positive integers (m,n) such that gcd(4m+2m+1,4n+2n+1)>1. We claim that (m,n) works if and only if either
- m and n are both even, or
- ν3(m)=ν3(n).
Proof of necessity. Suppose that p is a prime that divides both 4m+2m+1 and 4n+2n+1. If p=3, m and n must both be even. Henceforth assume that p=3. Let d be the order of 2 modulo p. Then, as p∣23m−1, we find that d∣3m. Furthermore, we cannot have p∣2m−1. Otherwise, by lifting the exponent (and p=3),
νp(23m−1)=νp(2m−1)+νp(3)=νp(2m−1)⟹νp(4m+2m+1)=0.
The previous two results imply ν3(d)=ν3(m)+1. Similarly, ν3(d)=ν3(n)+1, so ν3(m)=ν3(n).
Proof of sufficiency. If m and n are both even, then 3 divides both 4m+2m+1 and 4n+2n+1. If ν3(m)=ν3(n)=k, then we claim that 43k+23k+1 divides both 4m+2m+1 and 4n+2n+1. Indeed, note that as polynomials, x2+x+1 divides x2ℓ+xℓ+1 when 3∤ℓ. Plugging in x=23k with ℓ=m/3k and ℓ=n/3k yields the desired result.
Answer Extraction. We count pairs of odd integers (m,n) less than 80 with ν3(m)=ν3(n). Of all integers in the set {1,3,5,…,79}, there are 27, 9, 3, and 1 of them that have ν3 equal to 0, 1, 2, and 3, respectively. Hence, the answer is 272+92+32+12=820.