fi=sqrt(2+1/sqrt(2+1/sqrt(2+...)))
Pi = 6 - Sum_{n>=1} 1/((n+0,5)(n-0,5)) + 1/(2(n+0,25)(n-0,25))
a(n+1)=((n+2)/2)^4-(n/2)^4-a(n)
a(n+1)=int(3(n/2)^2+3(n/2)+1)+a(n)
a(0)=0
a(1)=1
a(n+2)=1,5n(n+3)+4+a(n)