Continued fraction closely related to the Rogers–Ramanujan identities
The Rogers–Ramanujan continued fraction is a continued fraction discovered by Rogers (1894) and independently by Srinivasa Ramanujan , and closely related to the Rogers–Ramanujan identities . It can be evaluated explicitly for a broad class of values of its argument.
Domain coloring representation of the convergent
A
400
(
q
)
/
B
400
(
q
)
{\displaystyle A_{400}(q)/B_{400}(q)}
of the function
q
−
1
/
5
R
(
q
)
{\displaystyle q^{-1/5}R(q)}
, where
R
(
q
)
{\displaystyle R(q)}
is the Rogers–Ramanujan continued fraction.
Definition [ ]
Representation of the approximation
q
1
/
5
A
400
(
q
)
/
B
400
(
q
)
{\displaystyle q^{1/5}A_{400}(q)/B_{400}(q)}
of the Rogers–Ramanujan continued fraction.
Given the functions
G
(
q
)
{\displaystyle G(q)}
and
H
(
q
)
{\displaystyle H(q)}
appearing in the Rogers–Ramanujan identities,
G
(
q
)
=
∑
n
=
0
∞
q
n
2
(
1
−
q
)
(
1
−
q
2
)
⋯
(
1
−
q
n
)
=
∑
n
=
0
∞
q
n
2
(
q
;
q
)
n
=
1
(
q
;
q
5
)
∞
(
q
4
;
q
5
)
∞
=
∏
n
=
1
∞
1
(
1
−
q
5
n
−
1
)
(
1
−
q
5
n
−
4
)
=
q
j
60
2
F
1
(
−
1
60
,
19
60
;
4
5
;
1728
j
)
=
q
(
j
−
1728
)
60
2
F
1
(
−
1
60
,
29
60
;
4
5
;
−
1728
j
−
1728
)
=
1
+
q
+
q
2
+
q
3
+
2
q
4
+
2
q
5
+
3
q
6
+
⋯
{\displaystyle {\begin{aligned}G(q)&=\sum _{n=0}^{\infty }{\frac {q^{n^{2}}}{(1-q)(1-q^{2})\cdots (1-q^{n})}}=\sum _{n=0}^{\infty }{\frac {q^{n^{2}}}{(q;q)_{n}}}={\frac {1}{(q;q^{5})_{\infty }(q^{4};q^{5})_{\infty }}}\\&=\prod _{n=1}^{\infty }{\frac {1}{(1-q^{5n-1})(1-q^{5n-4})}}\\&={\sqrt[{60}]{qj}}\,_{2}F_{1}\left(-{\tfrac {1}{60}},{\tfrac {19}{60}};{\tfrac {4}{5}};{\tfrac {1728}{j}}\right)\\&={\sqrt[{60}]{q\left(j-1728\right)}}\,_{2}F_{1}\left(-{\tfrac {1}{60}},{\tfrac {29}{60}};{\tfrac {4}{5}};-{\tfrac {1728}{j-1728}}\right)\\&=1+q+q^{2}+q^{3}+2q^{4}+2q^{5}+3q^{6}+\cdots \end{aligned}}}
and,
H
(
q
)
=
∑
n
=
0
∞
q
n
2
+
n
(
1
−
q
)
(
1
−
q
2
)
⋯
(
1
−
q
n
)
=
∑
n
=
0
∞
q
n
2
+
n
(
q
;
q
)
n
=
1
(
q
2
;
q
5
)
∞
(
q
3
;
q
5
)
∞
=
∏
n
=
1
∞
1
(
1
−
q
5
n
−
2
)
(
1
−
q
5
n
−
3
)
=
1
q
11
j
11
60
2
F
1
(
11
60
,
31
60
;
6
5
;
1728
j
)
=
1
q
11
(
j
−
1728
)
11
60
2
F
1
(
11
60
,
41
60
;
6
5
;
−
1728
j
−
1728
)
=
1
+
q
2
+
q
3
+
q
4
+
q
5
+
2
q
6
+
2
q
7
+
⋯
{\displaystyle {\begin{aligned}H(q)&=\sum _{n=0}^{\infty }{\frac {q^{n^{2}+n}}{(1-q)(1-q^{2})\cdots (1-q^{n})}}=\sum _{n=0}^{\infty }{\frac {q^{n^{2}+n}}{(q;q)_{n}}}={\frac {1}{(q^{2};q^{5})_{\infty }(q^{3};q^{5})_{\infty }}}\\&=\prod _{n=1}^{\infty }{\frac {1}{(1-q^{5n-2})(1-q^{5n-3})}}\\&={\frac {1}{\sqrt[{60}]{q^{11}j^{11}}}}\,_{2}F_{1}\left({\tfrac {11}{60}},{\tfrac {31}{60}};{\tfrac {6}{5}};{\tfrac {1728}{j}}\right)\\&={\frac {1}{\sqrt[{60}]{q^{11}\left(j-1728\right)^{11}}}}\,_{2}F_{1}\left({\tfrac {11}{60}},{\tfrac {41}{60}};{\tfrac {6}{5}};-{\tfrac {1728}{j-1728}}\right)\\&=1+q^{2}+q^{3}+q^{4}+q^{5}+2q^{6}+2q^{7}+\cdots \end{aligned}}}
OEIS : A003114 and OEIS : A003106 , respectively, where
(
a
;
q
)
∞
{\displaystyle (a;q)_{\infty }}
denotes the infinite q-Pochhammer symbol , j is the j-function , and 2 F1 is the hypergeometric function , then the Rogers–Ramanujan continued fraction is,
R
(
q
)
=
q
11
60
H
(
q
)
q
−
1
60
G
(
q
)
=
q
1
5
∏
n
=
1
∞
(
1
−
q
5
n
−
1
)
(
1
−
q
5
n
−
4
)
(
1
−
q
5
n
−
2
)
(
1
−
q
5
n
−
3
)
=
q
1
/
5
∏
n
=
1
∞
(
1
−
q
n
)
(
n
|
5
)
=
q
1
/
5
1
+
q
1
+
q
2
1
+
q
3
1
+
⋱
{\displaystyle {\begin{aligned}R(q)&={\frac {q^{\frac {11}{60}}H(q)}{q^{-{\frac {1}{60}}}G(q)}}=q^{\frac {1}{5}}\prod _{n=1}^{\infty }{\frac {(1-q^{5n-1})(1-q^{5n-4})}{(1-q^{5n-2})(1-q^{5n-3})}}=q^{1/5}\prod _{n=1}^{\infty }(1-q^{n})^{(n|5)}\\&={\cfrac {q^{1/5}}{1+{\cfrac {q}{1+{\cfrac {q^{2}}{1+{\cfrac {q^{3}}{1+\ddots }}}}}}}}\end{aligned}}}
(
n
|
m
)
{\displaystyle (n|m)}
denotes the Jacobi symbol.
Modular functions [ ]
If
q
=
e
2
π
i
τ
{\displaystyle q=e^{2\pi {\rm {i}}\tau }}
, then
q
−
1
60
G
(
q
)
{\displaystyle q^{-{\frac {1}{60}}}G(q)}
and
q
11
60
H
(
q
)
{\displaystyle q^{\frac {11}{60}}H(q)}
, as well as their quotient
R
(
q
)
{\displaystyle R(q)}
, are modular functions of
τ
{\displaystyle \tau }
. Since they have integral coefficients, the theory of complex multiplication implies that their values for
τ
{\displaystyle \tau }
an imaginary quadratic irrational are algebraic numbers that can be evaluated explicitly.
Examples [ ]
R
(
e
−
π
)
=
e
−
π
5
1
+
e
−
π
1
+
e
−
2
π
1
+
⋱
=
1
2
ϕ
(
5
−
ϕ
3
/
2
)
(
ϕ
3
/
2
+
5
4
)
{\displaystyle R{\big (}e^{-\pi }{\big )}={\cfrac {e^{-{\frac {\pi }{5}}}}{1+{\cfrac {e^{-\pi }}{1+{\cfrac {e^{-2\pi }}{1+\ddots }}}}}}={\frac {1}{2}}\phi ({\sqrt {5}}-\phi ^{3/2})(\phi ^{3/2}+{\sqrt[{4}]{5}})}
R
(
e
−
2
π
)
=
e
−
2
π
5
1
+
e
−
2
π
1
+
e
−
4
π
1
+
⋱
=
5
4
ϕ
1
/
2
−
ϕ
{\displaystyle R{\big (}e^{-2\pi }{\big )}={\cfrac {e^{-{\frac {2\pi }{5}}}}{1+{\cfrac {e^{-2\pi }}{1+{\cfrac {e^{-4\pi }}{1+\ddots }}}}}}={{\sqrt[{4}]{5}}\phi ^{1/2}-\phi }}
R
(
e
−
2
5
π
)
=
e
−
2
π
5
1
+
e
−
2
π
5
1
+
e
−
4
π
5
1
+
⋱
=
5
1
+
(
5
3
/
4
(
ϕ
−
1
)
5
/
2
−
1
)
1
/
5
−
ϕ
{\displaystyle R{\big (}e^{-2{\sqrt {5}}\pi }{\big )}={\cfrac {e^{-{\frac {2\pi }{\sqrt {5}}}}}{1+{\cfrac {e^{-2\pi {\sqrt {5}}}}{1+{\cfrac {e^{-4\pi {\sqrt {5}}}}{1+\ddots }}}}}}={\frac {\sqrt {5}}{1+{\big (}5^{3/4}(\phi -1)^{5/2}-1{\big )}^{1/5}}}-{\phi }}
where
ϕ
=
1
+
5
2
{\displaystyle \phi ={\frac {1+{\sqrt {5}}}{2}}}
is the golden ratio .
Relation to modular forms [ ]
R
(
q
)
{\displaystyle R(q)}
can be related to the Dedekind eta function , a modular form of weight 1/2, as,[1]
1
R
(
q
)
−
R
(
q
)
=
η
(
τ
5
)
η
(
5
τ
)
+
1
{\displaystyle {\frac {1}{R(q)}}-R(q)={\frac {\eta ({\frac {\tau }{5}})}{\eta (5\tau )}}+1}
1
R
5
(
q
)
−
R
5
(
q
)
=
[
η
(
τ
)
η
(
5
τ
)
]
6
+
11
{\displaystyle {\frac {1}{R^{5}(q)}}-R^{5}(q)=\left[{\frac {\eta (\tau )}{\eta (5\tau )}}\right]^{6}+11}
Therefore the Rogers-Ramanujan continued fraction can be expressed in terms of Jacobi theta function this way:
R
(
x
)
=
tan
⟨
1
2
arccot
{
1
2
[
ϑ
00
(
x
1
/
10
)
ϑ
01
(
x
1
/
10
)
ϑ
10
(
x
1
/
10
)
ϑ
00
(
x
5
/
2
)
ϑ
01
(
x
5
/
2
)
ϑ
10
(
x
5
/
2
)
]
1
/
3
+
1
2
}
⟩
{\displaystyle R(x)=\tan {\biggl \langle }{\frac {1}{2}}\operatorname {arccot} {\biggl \{}{\frac {1}{2}}{\biggl [}{\frac {\vartheta _{00}(x^{1/10})\vartheta _{01}(x^{1/10})\vartheta _{10}(x^{1/10})}{\vartheta _{00}(x^{5/2})\vartheta _{01}(x^{5/2})\vartheta _{10}(x^{5/2})}}{\biggr ]}^{1/3}+{\frac {1}{2}}{\biggr \}}{\biggr \rangle }}
R
(
x
)
=
tan
{
1
2
arctan
[
1
2
−
ϑ
01
(
x
)
2
2
ϑ
01
(
x
5
)
2
]
}
1
/
5
tan
{
1
2
arccot
[
1
2
−
ϑ
01
(
x
)
2
2
ϑ
01
(
x
5
)
2
]
}
2
/
5
{\displaystyle R(x)=\tan {\biggl \{}{\frac {1}{2}}\arctan {\biggl [}{\frac {1}{2}}-{\frac {\vartheta _{01}(x)^{2}}{2\vartheta _{01}(x^{5})^{2}}}{\biggr ]}{\biggr \}}^{1/5}\tan {\biggl \{}{\frac {1}{2}}\operatorname {arccot} {\biggl [}{\frac {1}{2}}-{\frac {\vartheta _{01}(x)^{2}}{2\vartheta _{01}(x^{5})^{2}}}{\biggr ]}{\biggr \}}^{2/5}}
R
(
x
)
=
tan
{
1
2
arctan
[
1
2
−
ϑ
01
(
x
1
/
2
)
2
2
ϑ
01
(
x
5
/
2
)
2
]
}
2
/
5
cot
{
1
2
arccot
[
1
2
−
ϑ
01
(
x
1
/
2
)
2
2
ϑ
01
(
x
5
/
2
)
2
]
}
1
/
5
{\displaystyle R(x)=\tan {\biggl \{}{\frac {1}{2}}\arctan {\biggl [}{\frac {1}{2}}-{\frac {\vartheta _{01}(x^{1/2})^{2}}{2\vartheta _{01}(x^{5/2})^{2}}}{\biggr ]}{\biggr \}}^{2/5}\cot {\biggl \{}{\frac {1}{2}}\operatorname {arccot} {\biggl [}{\frac {1}{2}}-{\frac {\vartheta _{01}(x^{1/2})^{2}}{2\vartheta _{01}(x^{5/2})^{2}}}{\biggr ]}{\biggr \}}^{1/5}}
Definition of the nome function :
q
(
k
)
=
exp
[
−
π
K
(
1
−
k
2
)
K
(
k
)
−
1
]
{\displaystyle q(k)=\exp[-\pi K({\sqrt {1-k^{2}}})K(k)^{-1}]}
The small letter k describes the elliptic modulus and the big letter K describes the complete elliptic integral of the first kind.
The continued fraction is related to the Jacobi elliptic functions as follows:
R
[
q
(
k
)
]
=
tan
1
/
5
{
1
2
arctan
y
}
tan
2
/
5
{
1
2
arccot
y
}
=
{
y
2
+
1
−
1
y
}
1
/
5
{
y
[
1
y
2
+
1
−
1
]
}
2
/
5
{\displaystyle R[q(k)]=\tan ^{1/5}{\biggl \{}{\frac {1}{2}}\arctan y{\biggr \}}\tan ^{2/5}{\biggl \{}{\frac {1}{2}}\operatorname {arccot} y{\biggr \}}=\left\{{\frac {{\sqrt {y^{2}+1}}-1}{y}}\right\}^{1/5}\left\{y\left[{\sqrt {{\frac {1}{y^{2}}}+1}}-1\right]\right\}^{2/5}}
with
y
=
2
k
2
sn
[
2
5
K
(
k
)
;
k
]
2
sn
[
4
5
K
(
k
)
;
k
]
2
5
−
k
2
sn
[
2
5
K
(
k
)
;
k
]
2
sn
[
4
5
K
(
k
)
;
k
]
2
.
{\displaystyle y={\frac {2k^{2}{\text{sn}}[{\tfrac {2}{5}}K(k);k]^{2}{\text{sn}}[{\tfrac {4}{5}}K(k);k]^{2}}{5-k^{2}{\text{sn}}[{\tfrac {2}{5}}K(k);k]^{2}{\text{sn}}[{\tfrac {4}{5}}K(k);k]^{2}}}.}
Relation to j-function [ ]
One formula involving the j-function and the Dedekind eta function is this:
j
(
τ
)
=
(
x
2
+
10
x
+
5
)
3
x
{\displaystyle j(\tau )={\frac {(x^{2}+10x+5)^{3}}{x}}}
where
x
=
[
5
η
(
5
τ
)
η
(
τ
)
]
6
{\displaystyle x=\left[{\frac {{\sqrt {5}}\,\eta (5\tau )}{\eta (\tau )}}\right]^{6}}
Eliminating the eta quotient
x
{\displaystyle x}
, one can then express j (τ ) in terms of
r
=
R
(
q
)
{\displaystyle r=R(q)}
as,
j
(
τ
)
=
−
(
r
20
−
228
r
15
+
494
r
10
+
228
r
5
+
1
)
3
r
5
(
r
10
+
11
r
5
−
1
)
5
j
(
τ
)
−
1728
=
−
(
r
30
+
522
r
25
−
10005
r
20
−
10005
r
10
−
522
r
5
+
1
)
2
r
5
(
r
10
+
11
r
5
−
1
)
5
{\displaystyle {\begin{aligned}&j(\tau )=-{\frac {(r^{20}-228r^{15}+494r^{10}+228r^{5}+1)^{3}}{r^{5}(r^{10}+11r^{5}-1)^{5}}}\\[6pt]&j(\tau )-1728=-{\frac {(r^{30}+522r^{25}-10005r^{20}-10005r^{10}-522r^{5}+1)^{2}}{r^{5}(r^{10}+11r^{5}-1)^{5}}}\end{aligned}}}
where the numerator and denominator are polynomial invariants of the icosahedron . Using the modular equation between
R
(
q
)
{\displaystyle R(q)}
and
R
(
q
5
)
{\displaystyle R(q^{5})}
, one finds that,
j
(
5
τ
)
=
−
(
r
20
+
12
r
15
+
14
r
10
−
12
r
5
+
1
)
3
r
25
(
r
10
+
11
r
5
−
1
)
{\displaystyle j(5\tau )=-{\frac {(r^{20}+12r^{15}+14r^{10}-12r^{5}+1)^{3}}{r^{25}(r^{10}+11r^{5}-1)}}}
let
z
=
r
5
−
1
r
5
{\displaystyle z=r^{5}-{\frac {1}{r^{5}}}}
, then
j
(
5
τ
)
=
−
(
z
2
+
12
z
+
16
)
3
z
+
11
{\displaystyle j(5\tau )=-{\frac {\left(z^{2}+12z+16\right)^{3}}{z+11}}}
where
z
∞
=
−
[
5
η
(
25
τ
)
η
(
5
τ
)
]
6
−
11
,
z
0
=
−
[
η
(
τ
)
η
(
5
τ
)
]
6
−
11
,
z
1
=
[
η
(
5
τ
+
2
5
)
η
(
5
τ
)
]
6
−
11
,
z
2
=
−
[
η
(
5
τ
+
4
5
)
η
(
5
τ
)
]
6
−
11
,
z
3
=
[
η
(
5
τ
+
6
5
)
η
(
5
τ
)
]
6
−
11
,
z
4
=
−
[
η
(
5
τ
+
8
5
)
η
(
5
τ
)
]
6
−
11
{\displaystyle {\begin{aligned}&z_{\infty }=-\left[{\frac {{\sqrt {5}}\,\eta (25\tau )}{\eta (5\tau )}}\right]^{6}-11,\ z_{0}=-\left[{\frac {\eta (\tau )}{\eta (5\tau )}}\right]^{6}-11,\ z_{1}=\left[{\frac {\eta ({\frac {5\tau +2}{5}})}{\eta (5\tau )}}\right]^{6}-11,\\[6pt]&z_{2}=-\left[{\frac {\eta ({\frac {5\tau +4}{5}})}{\eta (5\tau )}}\right]^{6}-11,\ z_{3}=\left[{\frac {\eta ({\frac {5\tau +6}{5}})}{\eta (5\tau )}}\right]^{6}-11,\ z_{4}=-\left[{\frac {\eta ({\frac {5\tau +8}{5}})}{\eta (5\tau )}}\right]^{6}-11\end{aligned}}}
which in fact is the j-invariant of the elliptic curve ,
y
2
+
(
1
+
r
5
)
x
y
+
r
5
y
=
x
3
+
r
5
x
2
{\displaystyle y^{2}+(1+r^{5})xy+r^{5}y=x^{3}+r^{5}x^{2}}
parameterized by the non-cusp points of the modular curve
X
1
(
5
)
{\displaystyle X_{1}(5)}
.
Functional equation [ ]
For convenience, one can also use the notation
r
(
τ
)
=
R
(
q
)
{\displaystyle r(\tau )=R(q)}
when q = e2πiτ . While other modular functions like the j-invariant satisfies,
j
(
−
1
τ
)
=
j
(
τ
)
{\displaystyle j(-{\tfrac {1}{\tau }})=j(\tau )}
and the Dedekind eta function has,
η
(
−
1
τ
)
=
−
i
τ
η
(
τ
)
{\displaystyle \eta (-{\tfrac {1}{\tau }})={\sqrt {-i\tau }}\,\eta (\tau )}
the functional equation of the Rogers–Ramanujan continued fraction involves[2] the golden ratio
ϕ
{\displaystyle \phi }
,
r
(
−
1
τ
)
=
1
−
ϕ
r
(
τ
)
ϕ
+
r
(
τ
)
{\displaystyle r(-{\tfrac {1}{\tau }})={\frac {1-\phi \,r(\tau )}{\phi +r(\tau )}}}
Incidentally,
r
(
7
+
i
10
)
=
i
{\displaystyle r({\tfrac {7+i}{10}})=i}
Modular equations [ ]
There are modular equations between
R
(
q
)
{\displaystyle R(q)}
and
R
(
q
n
)
{\displaystyle R(q^{n})}
. Elegant ones for small prime n are as follows.[3]
For
n
=
2
{\displaystyle n=2}
, let
u
=
R
(
q
)
{\displaystyle u=R(q)}
and
v
=
R
(
q
2
)
{\displaystyle v=R(q^{2})}
, then
v
−
u
2
=
(
v
+
u
2
)
u
v
2
.
{\displaystyle v-u^{2}=(v+u^{2})uv^{2}.}
For
n
=
3
{\displaystyle n=3}
, let
u
=
R
(
q
)
{\displaystyle u=R(q)}
and
v
=
R
(
q
3
)
{\displaystyle v=R(q^{3})}
, then
(
v
−
u
3
)
(
1
+
u
v
3
)
=
3
u
2
v
2
.
{\displaystyle (v-u^{3})(1+uv^{3})=3u^{2}v^{2}.}
For
n
=
5
{\displaystyle n=5}
, let
u
=
R
(
q
)
{\displaystyle u=R(q)}
and
v
=
R
(
q
5
)
{\displaystyle v=R(q^{5})}
, then
(
v
4
−
3
v
3
+
4
v
2
−
2
v
+
1
)
v
=
(
v
4
+
2
v
3
+
4
v
2
+
3
v
+
1
)
u
5
.
{\displaystyle (v^{4}-3v^{3}+4v^{2}-2v+1)v=(v^{4}+2v^{3}+4v^{2}+3v+1)u^{5}.}
For
n
=
11
{\displaystyle n=11}
, let
u
=
R
(
q
)
{\displaystyle u=R(q)}
and
v
=
R
(
q
11
)
{\displaystyle v=R(q^{11})}
, then
u
v
(
u
10
+
11
u
5
−
1
)
(
v
10
+
11
v
5
−
1
)
=
(
u
−
v
)
12
.
{\displaystyle uv(u^{10}+11u^{5}-1)(v^{10}+11v^{5}-1)=(u-v)^{12}.}
Regarding
n
=
5
{\displaystyle n=5}
, note that
v
10
+
11
v
5
−
1
=
(
v
2
+
v
−
1
)
(
v
4
−
3
v
3
+
4
v
2
−
2
v
+
1
)
(
v
4
+
2
v
3
+
4
v
2
+
3
v
+
1
)
.
{\displaystyle v^{10}+11v^{5}-1=(v^{2}+v-1)(v^{4}-3v^{3}+4v^{2}-2v+1)(v^{4}+2v^{3}+4v^{2}+3v+1).}
Derivatives [ ]
For
|
q
|
<
1
{\displaystyle |q|<1}
, the first order derivative of
R
(
q
)
{\displaystyle R(q)}
given in terms of the Euler function
f
(
−
q
)
:=
∏
n
=
1
∞
(
1
−
q
n
)
{\displaystyle f(-q):=\prod _{n=1}^{\infty }(1-q^{n})}
is [4]
R
′
(
q
)
=
5
−
1
q
−
5
/
6
f
(
−
q
)
4
R
(
q
)
R
(
q
)
−
5
−
11
−
R
(
q
)
5
6
.
{\displaystyle R'(q)=5^{-1}q^{-5/6}f(-q)^{4}R(q){\sqrt[{6}]{R(q)^{-5}-11-R(q)^{5}}}.}
Setting
q
=
exp
(
2
π
i
z
)
{\displaystyle q=\exp(2\pi iz)}
, where
Im
(
z
)
>
0
{\displaystyle \operatorname {Im} (z)>0}
, the function
f
(
−
q
)
{\displaystyle f(-q)}
is related with the Dedekind eta function
η
(
z
)
=
q
1
/
24
f
(
−
q
)
.
{\displaystyle \eta (z)=q^{1/24}f(-q).}
Ramanujan has stated that, for real
q
{\displaystyle q}
, with
|
q
|
<
1
{\displaystyle |q|<1}
, we have [5]
R
(
q
)
=
5
−
1
2
exp
(
1
5
∫
1
q
f
(
−
t
)
5
t
f
(
−
t
5
)
d
t
)
.
{\displaystyle R(q)={\frac {{\sqrt {5}}-1}{2}}\exp \left({\frac {1}{5}}\int _{1}^{q}{\frac {f(-t)^{5}}{tf(-t^{5})}}dt\right).}
By taking the logarithmic derivative of the above relation and applying analytic continuation we get
R
′
(
q
)
=
f
(
−
q
)
5
5
q
f
(
−
q
5
)
R
(
q
)
,
{\displaystyle R'(q)={\frac {f(-q)^{5}}{5qf(-q^{5})}}R(q),}
with validity to all complex
q
{\displaystyle q}
with
|
q
|
<
1
{\displaystyle |q|<1}
.
Evaluations [ ]
Assume
q
=
e
−
π
r
{\displaystyle q=e^{-\pi {\sqrt {r}}}}
,
r
>
0
{\displaystyle r>0}
and
0
<
k
r
<
1
{\displaystyle 0<k_{r}<1}
is the root of the equation
K
(
k
r
′
)
/
K
(
k
r
)
=
r
{\displaystyle K\left(k'_{r}\right)/K(k_{r})={\sqrt {r}}}
,
k
r
′
=
1
−
k
r
2
{\displaystyle k'_{r}={\sqrt {1-k_{r}^{2}}}}
(here
k
r
{\displaystyle k_{r}}
is the elliptic singular modulus associated with the nome (mathematics) and
K
(
x
)
=
π
/
2
⋅
2
F
1
(
1
/
2
,
1
/
2
;
1
;
x
2
)
{\displaystyle K(x)=\pi /2\cdot {}_{2}F_{1}(1/2,1/2;1;x^{2})}
is the complete elliptic integral of the first kind also see [6] ,[7] ,[8] ). Then if
a
r
=
(
k
r
′
k
25
r
′
)
2
k
r
k
25
r
M
5
(
r
)
−
3
,
{\displaystyle a_{r}=\left({\frac {k'_{r}}{k'_{25r}}}\right)^{2}{\sqrt {\frac {k_{r}}{k_{25r}}}}M_{5}(r)^{-3},}
we get [9]
R
(
q
)
=
(
−
11
2
−
a
r
2
+
1
2
125
+
22
a
r
+
a
r
2
)
1
/
5
,
{\displaystyle R(q)=\left(-{\frac {11}{2}}-{\frac {a_{r}}{2}}+{\frac {1}{2}}{\sqrt {125+22a_{r}+a_{r}^{2}}}\right)^{1/5},}
where
M
5
(
r
)
{\displaystyle M_{5}(r)}
is root of
(
5
x
−
1
)
5
(
1
−
x
)
=
256
(
k
r
k
r
′
)
2
x
.
{\displaystyle (5x-1)^{5}(1-x)=256(k_{r}k'_{r})^{2}x.}
Also
R
′
(
q
)
=
2
4
/
3
(
k
r
)
5
/
12
(
k
r
′
)
5
/
3
5
(
k
25
r
)
1
/
12
(
k
25
r
′
)
1
/
3
M
5
(
r
)
R
(
q
)
K
(
k
r
)
2
π
2
q
.
{\displaystyle R'(q)={\frac {2^{4/3}(k_{r})^{5/12}(k'_{r})^{5/3}}{5(k_{25r})^{1/12}(k'_{25r})^{1/3}{\sqrt {M_{5}(r)}}}}R(q){\frac {K(k_{r})^{2}}{\pi ^{2}q}}.}
Examples of evaluations of
R
(
e
−
π
r
)
{\displaystyle R(e^{-\pi {\sqrt {r}}})}
and
R
′
(
e
−
π
r
)
{\displaystyle R'(e^{-\pi {\sqrt {r}}})}
, can be found in tables in the literature. For instance, if we set
r
=
1
{\displaystyle r=1}
, then
k
1
=
1
/
2
{\displaystyle k_{1}=1/{\sqrt {2}}}
,
k
1
′
=
1
/
2
{\displaystyle k'_{1}=1/{\sqrt {2}}}
,
M
5
(
1
)
=
5
−
1
(
2
+
5
)
{\displaystyle M_{5}(1)=5^{-1}(2+{\sqrt {5}})}
and [8] pg.334:
k
25
2
=
2
−
1
(
5
−
2
)
2
(
3
−
2
⋅
5
1
/
4
)
2
{\displaystyle k_{25}^{2}=2^{-1}({\sqrt {5}}-2)^{2}(3-2\cdot 5^{1/4})^{2}}
, [8] pg.331:
K
(
k
1
)
=
Γ
(
1
/
4
)
2
/
(
4
π
)
{\displaystyle K(k_{1})=\Gamma (1/4)^{2}/(4{\sqrt {\pi }})}
, where
Γ
(
x
)
{\displaystyle \Gamma (x)}
is the Euler gamma function .
Other results [ ]
Ramanujan found many other interesting results regarding
R
(
q
)
{\displaystyle R(q)}
.[10] Let
a
,
b
∈
R
+
{\displaystyle a,b\in \mathbb {R} ^{+}}
,
u
=
R
(
e
−
2
a
)
{\displaystyle u=R(e^{-2a})}
,
v
=
R
(
e
−
2
b
)
{\displaystyle v=R(e^{-2b})}
, and
ϕ
{\displaystyle \phi }
as the golden ratio .
If
a
b
=
π
2
{\displaystyle ab=\pi ^{2}}
, then
(
u
+
ϕ
)
(
v
+
ϕ
)
=
5
ϕ
.
{\displaystyle (u+\phi )(v+\phi )={\sqrt {5}}\,\phi .}
If
5
a
b
=
π
2
{\displaystyle 5ab=\pi ^{2}}
, then
(
u
5
+
ϕ
5
)
(
v
5
+
ϕ
5
)
=
5
5
ϕ
5
.
{\displaystyle (u^{5}+\phi ^{5})(v^{5}+\phi ^{5})=5{\sqrt {5}}\,\phi ^{5}.}
The powers of
R
(
q
)
{\displaystyle R(q)}
also can be expressed in unusual ways. For its cube ,
R
3
(
q
)
=
α
β
{\displaystyle R^{3}(q)={\frac {\alpha }{\beta }}}
where,
α
=
∑
n
=
0
∞
q
2
n
1
−
q
5
n
+
2
−
∑
n
=
0
∞
q
3
n
+
1
1
−
q
5
n
+
3
{\displaystyle \alpha =\sum _{n=0}^{\infty }{\frac {q^{2n}}{1-q^{5n+2}}}-\sum _{n=0}^{\infty }{\frac {q^{3n+1}}{1-q^{5n+3}}}}
β
=
∑
n
=
0
∞
q
n
1
−
q
5
n
+
1
−
∑
n
=
0
∞
q
4
n
+
3
1
−
q
5
n
+
4
{\displaystyle \beta =\sum _{n=0}^{\infty }{\frac {q^{n}}{1-q^{5n+1}}}-\sum _{n=0}^{\infty }{\frac {q^{4n+3}}{1-q^{5n+4}}}}
For its fifth power, let
w
=
R
(
q
)
R
2
(
q
2
)
{\displaystyle w=R(q)R^{2}(q^{2})}
, then,
R
5
(
q
)
=
w
(
1
−
w
1
+
w
)
2
,
R
5
(
q
2
)
=
w
2
(
1
+
w
1
−
w
)
{\displaystyle R^{5}(q)=w\left({\frac {1-w}{1+w}}\right)^{2},\;\;R^{5}(q^{2})=w^{2}\left({\frac {1+w}{1-w}}\right)}
Quintic equations [ ]
The general quintic equation in Bring-Jerrard Form can be solved in terms of Rogers-Ramanujan continued fraction.
For every real value a > 1 this equation can be solved with the function R(q) and the elliptic nome q(k):
x
5
−
5
x
−
4
a
=
0
{\displaystyle x^{5}-5x-4a=0}
To solve this equation, the elliptic modulus must be determined according to the following pattern:
k
=
tan
[
1
4
π
−
1
4
arccsc
(
a
2
)
]
=
(
a
2
+
1
−
|
a
|
)
(
|
a
|
+
a
2
−
1
)
{\displaystyle k=\tan[{\tfrac {1}{4}}\pi -{\tfrac {1}{4}}\operatorname {arccsc}(a^{2})]=({\sqrt {a^{2}+1}}-|a|)(|a|+{\sqrt {a^{2}-1}})}
Then this is the real solution of this quintic equation:
x
=
2
−
{
1
−
R
[
q
(
k
)
]
}
{
1
+
R
[
q
(
k
)
2
]
}
R
[
q
(
k
)
]
R
[
q
(
k
)
2
]
4
cot
⟨
4
arctan
{
R
[
q
(
k
)
]
R
[
q
(
k
)
2
]
2
}
⟩
−
3
4
=
2
−
{
1
−
R
[
q
(
k
)
]
}
{
1
+
R
[
q
(
k
)
2
]
}
R
[
q
(
k
)
]
R
[
q
(
k
)
2
]
2
R
[
q
(
k
)
]
R
[
q
(
k
)
2
]
2
−
1
+
2
R
[
q
(
k
)
]
R
[
q
(
k
)
2
]
2
+
1
+
1
R
[
q
(
k
)
]
R
[
q
(
k
)
2
]
2
−
R
[
q
(
k
)
]
R
[
q
(
k
)
2
]
2
−
3
4
{\displaystyle {\begin{aligned}x&={\frac {2-{\bigl \{}1-R[q(k)]{\bigr \}}{\bigl \{}1+R[q(k)^{2}]{\bigr \}}}{{\sqrt {R[q(k)]R[q(k)^{2}]}}{\sqrt[{4}]{4\cot \langle 4\arctan\{R[q(k)]R[q(k)^{2}]^{2}\}\rangle -3}}}}\\&={\frac {2-{\bigl \{}1-R[q(k)]{\bigr \}}{\bigl \{}1+R[q(k)^{2}]{\bigr \}}}{{\sqrt {R[q(k)]R[q(k)^{2}]}}{\sqrt[{4}]{{\frac {2}{R[q(k)]R[q(k)^{2}]^{2}-1}}+{\frac {2}{R[q(k)]R[q(k)^{2}]^{2}+1}}+{\frac {1}{R[q(k)]R[q(k)^{2}]^{2}}}-R[q(k)]R[q(k)^{2}]^{2}-3}}}}\end{aligned}}}
For example, the following equation has the following real solution:
x
5
−
x
−
1
=
0
{\displaystyle x^{5}-x-1=0}
(
5
4
x
)
5
−
5
(
5
4
x
)
−
4
(
5
4
5
4
)
=
0
{\displaystyle ({\sqrt[{4}]{5}}x)^{5}-5({\sqrt[{4}]{5}}x)-4({\tfrac {5}{4}}{\sqrt[{4}]{5}})=0}
x
=
2
−
{
1
−
R
[
q
(
5
5
/
4
+
25
5
−
16
25
5
+
16
+
5
5
/
4
)
]
}
{
1
+
R
[
q
(
5
5
/
4
+
25
5
−
16
25
5
+
16
+
5
5
/
4
)
2
]
}
R
[
q
(
5
5
/
4
+
25
5
−
16
25
5
+
16
+
5
5
/
4
)
]
R
[
q
(
5
5
/
4
+
25
5
−
16
25
5
+
16
+
5
5
/
4
)
2
]
20
cot
⟨
4
arctan
{
R
[
q
(
5
5
/
4
+
25
5
−
16
25
5
+
16
+
5
5
/
4
)
]
R
[
q
(
5
5
/
4
+
25
5
−
16
25
5
+
16
+
5
5
/
4
)
2
]
2
}
⟩
−
15
4
{\displaystyle x={\frac {2-{\biggl \{}1-R{\biggl [}q{\biggl (}{\tfrac {5^{5/4}+{\sqrt {25{\sqrt {5}}-16}}}{{\sqrt {25{\sqrt {5}}+16}}+5^{5/4}}}{\biggr )}{\biggr ]}{\biggr \}}{\biggl \{}1+R{\biggl [}q{\biggl (}{\tfrac {5^{5/4}+{\sqrt {25{\sqrt {5}}-16}}}{{\sqrt {25{\sqrt {5}}+16}}+5^{5/4}}}{\biggr )}^{2}{\biggr ]}{\biggr \}}}{{\sqrt {R{\biggl [}q{\biggl (}{\tfrac {5^{5/4}+{\sqrt {25{\sqrt {5}}-16}}}{{\sqrt {25{\sqrt {5}}+16}}+5^{5/4}}}{\biggr )}{\biggr ]}R{\biggl [}q{\biggl (}{\tfrac {5^{5/4}+{\sqrt {25{\sqrt {5}}-16}}}{{\sqrt {25{\sqrt {5}}+16}}+5^{5/4}}}{\biggr )}^{2}{\biggr ]}}}{\sqrt[{4}]{20\cot {\biggl \langle }4\arctan {\biggl \{}R{\biggl [}q{\biggl (}{\tfrac {5^{5/4}+{\sqrt {25{\sqrt {5}}-16}}}{{\sqrt {25{\sqrt {5}}+16}}+5^{5/4}}}{\biggr )}{\biggr ]}R{\biggl [}q{\biggl (}{\tfrac {5^{5/4}+{\sqrt {25{\sqrt {5}}-16}}}{{\sqrt {25{\sqrt {5}}+16}}+5^{5/4}}}{\biggr )}^{2}{\biggr ]}^{2}{\biggr \}}{\biggr \rangle }-15}}}}}
This constant is roughly 1.1673 and it can not be represented by elementary root expressions.
References [ ]
^ Duke, W. "Continued Fractions and Modular Functions", https://www.math.ucla.edu/~wdduke/preprints/bams4.pdf
^ Duke, W. "Continued Fractions and Modular Functions" (p.9)
^ Berndt, B. et al. "The Rogers–Ramanujan Continued Fraction", http://www.math.uiuc.edu/~berndt/articles/rrcf.pdf
^ N.Bagis, L.Glasser. (2009). "Integrals related with Rogers Ramanujan continued fraction and q-products". arXiv:0904.1641. https://arxiv.org/ftp/arxiv/papers/0904/0904.1641.pdf
^ G.Andrews. (1979). Amer. Math. Monthly.Vol 86. pg 89-108.
^ J.M. Borwein and P.B. Borwein. (1987). "Pi and the AGM: A Study in Analytic Number Theory and Computational Complexity", Wiley, New York.
^ D. Broadhurst. (2008). 'Solutions by Radicals at Singular Values
k
N
{\displaystyle k_{N}}
from New Class Invariants for
N
≡
3
m
o
d
8
{\displaystyle N\equiv 3mod8}
'. arXiv:0807.2976 (math-phy).
^ J.M. Borwein, M.L. Glasser, R.C. McPhedran, J.G. Wan, I.J. Zucker. (2013). 'Lattice Sums Then and Now'. Cambridge University Press. New York.
^ Nikos Bagis. (2014)."The complete evaluation of Rogers-Ramanujan and other continued fractions with elliptic functions". arXiv:1008.1304v2[math.GM] https://arxiv.org/pdf/1008.1304.pdf
^ Berndt, B. et al. "The Rogers–Ramanujan Continued Fraction"
Rogers, L. J. (1894), "Second Memoir on the Expansion of certain Infinite Products" , Proc. London Math. Soc. , s1-25 (1): 318–343, doi :10.1112/plms/s1-25.1.318
Berndt, B. C.; Chan, H. H.; Huang, S. S.; Kang, S. Y.; Sohn, J.; Son, S. H. (1999), "The Rogers–Ramanujan continued fraction" (PDF) , Journal of Computational and Applied Mathematics , 105 (1–2): 9–24, doi :10.1016/S0377-0427(99)00033-3
External links [ ]