[seqfan] Suggested method to generate A004080 b-file

[David Wilson]

For you math package freaks out there:

Equation (15) on http://mathworld.wolfram.com/HarmonicNumber.html

gives a very good estimate of H_k, the kth harmonic number, specifically

ln(k+1/2)+gamma+1/(24(k+1)^2) < H_k < ln(k+1/2)+gamma+1/(24k^2) [1]

where gamma is the Euler-Mascheroni constant, which is known very 
accurately (to billions of digits).

A004080(n) is the smallest k with H_k > n. The interval in [1], which is 
O(k^-3) in length, is extremely unlikely to straddle an integer for the 
large k in question. It should be sufficient to decide whether a given 
H_k is less than or greater than integer n. This is the basis of a 
binary or adaptive search algorithm for A004080(n), which would help us 
generate a b-file for that sequence.

A good test would be A004080(100)
= A099618(2)
= 15092688622113788323693563264538101449859497