Transformation of Elliptic Functions
The relation y=xM(p−1)/2∏s=11−x2sn24sω1−k2x2sn24sω and other variants of it 1−y=(1−x)(p−1)/2∏s=1(1−xsn(K−4sω))21−k2x2sn24sω1+y=(1+x)(p−1)/2∏s=1(1+xsn(K−4sω))21−k2x2sn24sω1−ly=(1−kx)(p−1)/2∏s=1(1−kxsn(K−4sω))21−k2x2sn24sω1+ly=(1+kx)(p−1)/2∏s=1(1+kxsn(K−4sω))21−k2x2sn24sω as described in previous post lead to the differential equation Mdy√(1−y2)(1−l2y2)=dx√(1−x2)(1−k2x2) when M=(−1)(p−1)/2(p−1)/2∏s=1(sn(K−4sω)sn4sω)2 and l=kp(p−1)/2∏s=1sn4(K−4sω)Putting x=sn(u,k) in the differential equation we see that y=sn(u/M,l). Hence the relation between x and y in effect expresses sn(u/M,l) in terms of elliptic functions of a different but related modulus k.
Thus we obtain sn(uM,l)=snuM(p−1)/2∏s=11−sn2usn24sω1−k2sn2usn24sω Using the expressions of 1−y,1+y,1−ly,1+ly it is easy to see that √1−y2=√1−x2(p−1)/2∏s=11−x2sn2(K−4sω)1−k2x2sn24sω√1−l2y2=√1−k2x2(p−1)/2∏s=11−k2x2sn2(K−4sω)1−k2x2sn24sω In the last relation if we put x=1 so that y=1 we get the expression for l′ as l′=k′(p−1)/2∏s=11−k2sn2(K−4sω)1−k2sn24sω=k′(p−1)/2∏s=1dn2(K−4sω)dn24sω=k′(p−1)/2∏s=1(k′dn24sω)2 and so l′=k′p(p−1)/2∏s=1dn44sω Putting x=snu in the expressions for √1−y2,√1−l2y2 we get the following results: cn(uM,l)=cnu(p−1)/2∏s=11−sn2usn2(K−4sω)1−k2sn2usn24sωdn(uM,l)=dnu(p−1)/2∏s=11−k2sn2usn2(K−4sω)1−k2sn2usn24sω The above formulas remain unchanged if M is replaced by −M and hence in the above formulas we can keep M=(p−1)/2∏s=1(sn(K−4sω)sn4sω)2 Now the values of sn(u+4ω),sn(u+8ω),…,sn(u+2(p−1)ω) are the same as the values sn(u+2ω),sn(u+4ω),…,sn(u+(p−1)ω) except for the order of terms and sign of each term. It is best to check this by putting some actual odd value of p (like 3, 5 etc). But upon squaring these values we see that the problem of sign no longer exists and therefore in each of the above relations we can actually replace 4s by 2s (by putting u=0 and u=K and noting that sn(K+α)=sn(K−α). Thus we arrive at the following: sn(uM,l)=snuM(p−1)/2∏s=11−sn2usn22sω1−k2sn2usn22sωcn(uM,l)=cnu(p−1)/2∏s=11−sn2usn2(K−2sω)1−k2sn2usn22sωdn(uM,l)=dnu(p−1)/2∏s=11−k2sn2usn2(K−2sω)1−k2sn2usn22sωM=(p−1)/2∏s=1(sn(K−2sω)sn2sω)2l=kp(p−1)/2∏s=1sn4(K−2sω)l′=k′p(p−1)/2∏s=1dn42sω In the development of these formulas so far we have assumed that ω=(mK+m′iK′)/p where m,m′ are relatively prime to each other, but we have not given any specific values to m,m′. It turns that by giving different values to m and m′ we arrive at different transformations. Out of all these transformations two are real i.e. the relation between x and y is expressed using real numbers as coefficients. We are going to study these two transformations next.
Jacobi's First Real Transformation
By choosing m=1,m′=0 we get ω=K/p which is a real number and from this we instantly see that all the numbers involved in the formulas mentioned above are real numbers. The corresponding formulas are as follows: sn(uM,l)=snuM(p−1)/2∏s=11−sn2usn22sKp1−k2sn2usn22sKpcn(uM,l)=cnu(p−1)/2∏s=11−sn2usn2(K−2sKp)1−k2sn2usn22sKpdn(uM,l)=dnu(p−1)/2∏s=11−k2sn2usn2(K−2sKp)1−k2sn2usn22sKpM=(p−1)/2∏s=1(sn(K−2sKp)sn2sKp)2l=kp(p−1)/2∏s=1sn4(K−2sKp)l′=k′p(p−1)/2∏s=1dn42sKp It is easy to see that the least positive value of u for which the right side of (1) vanishes is u=2K/p and the left side of this equation vanishes when u/M=2L. Equating these two values of u we get the relation K/pM=L.Jacobi's First Complementary Transformation
Next we need to recast the above formulas in another form by using the identities: 1−sn2usn2v1−k2sn2usn2v=−sn(u+v)sn(u−v)sn2v1−sn2usn2(K−v)1−k2sn2usn2v=cn(u+v)cn(u−v)cn2v1−k2sn2usn2(K−v)1−k2sn2usn2v=dn(u+v)dn(u−v)dn2v On applying these identities to the formulas (1) and using (4) to replace M we get sn(uM,l)=(−1)(p−1)/2snu(p−1)/2∏s=1(sn2sKpsn(K−2sKp))2(p−1)/2∏s=1sn(u+2sKp)sn(u−2sKp)sn22sKp and using equation (5) we get sn(uM,l)=(−1)(p−1)/2√kpl(p−1)/2∏s=−(p−1)/2sn(u+2sKp) and similarly cn(uM,l)=√l′kplk′p(p−1)/2∏s=−(p−1)/2cn(u+2sKp)dn(uM,l)=√l′k′p(p−1)/2∏s=−(p−1)/2dn(u+2sKp) Dividing the equation (7) by equation (8) we get sc(uM,l)=(−1)(p−1)/2√k′pl′(p−1)/2∏s=−(p−1)/2sc(u+2sKp) Replacing u by iu in the above equation and noting that sc(iu,k)=isn(u,k′) we get sn(uM,l′)=√k′pl′(p−1)/2∏s=−(p−1)/2sn(u−2siKp,k′)=√k′pl′sn(u,k′)(p−1)/2∏s=1sn(u+2siKp,k′)sn(u−2siKp,k′)=(−1)(p−1)/2(p−1)/2∏s=1sn2(2siKp,k′)sn(u,k′)(p−1)/2∏s=11−sn2(u,k′)sn2(2siKp,k′)1−k′2sn2(u,k′)sn2(2siKp,k′) The factor on the right side which is independent of u must be 1/M as can be easily seen when we divide both sides by sn(u,k′) and take limits as u→0.Thus we finally obtain sn(uM,l′)=sn(u,k′)M(p−1)/2∏s=11+sn2(u,k′)sc2(2sKp,k)1+k′2sn2(u,k′)sc2(2sKp,k) The only factor on the right side which can vanish is sn(u,k′) and the smallest value for which this vanishes is u=2K′. The left side vanishes when u/M=2L′ and therefore upon equating these two values of u we get K′/M=L′.
We have thus derived the two relations K/pM=L and K′/M=L′ from which we obtain the fundamental relation L′/L=pK′/K. From this it also follows that the value of l occurring in these equations is actually less than k.
In the next post we will describe one more transformation which is also real and it leads to a value of l which is greater than k (which will be denoted by l1) and in this case K′/K=pL′1/L1.
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