OFFSET
1,5
COMMENTS
Machin's formula states that Pi/4 = 4*arctan(1/5) - arctan(1/239). An approximation of Pi can be found by computing this using a Taylor series approximation of arctan. If n terms are used in the approximation of arctan(1/5), then a(n) is the least number of terms that can be used in the approximation of arctan(1/239) to get the largest possible number of correct digits of Pi.
LINKS
EXAMPLE
When using 5 terms in the Taylor series expansion of arctan(1/5) and 2 terms in the expansion of arctan(1/239), Machin's formula gives 3.141592682405... which is correct to 7 decimal places. If more than 2 terms are used in the second expansion, no more correct digits are obtained. If fewer than 2 terms are used, fewer correct digits will be obtained. Therefore a(5) = 2.
CROSSREFS
A350799(n) is the number of decimal places that will be correct when n terms are used for arctan(1/5) and a(n) terms are used for arctan(1/239).
KEYWORD
nonn,base,more
AUTHOR
Matthew Scroggs, Jan 18 2022
STATUS
approved