arcsinc( 1-y ), for small y

[Albert Chan]

This new formula is slightly more accurate, and just as simple:

For small y: arcsinc( 1-y ) ~ √(6y) / (1 - 0.15 y)

arcsinc(1 - 1/5281) ~ √(6/5281) / (1 - 0.15/5281) = 0.033707 75877

Example:
x = 0.1 radian
y = 1 - sin(x) / x = 0.001665833532

Old formula, x ~ √(6y) * (1 + 0.15 y) = 0.099999 98408
New formula, x ~ √(6y) / (1 - 0.15 y) = 0.099999 99033

Update: newer formula from post #11

x ~\(\large \sqrt{6y ÷ \sqrt{1 + 0.6 y}} \) = 0.099999 99997 // almost round-trip!