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mathematics of computationvolume 61, number 203july 1993, pages 83-95
A FUNDAMENTAL MODULAR IDENTITYAND SOME APPLICATIONS
RICHARD BLECKSMITH, JOHN BRILLHART, AND IRVING GERST
Dedicated to the memory ofD. H. Lehmer
Abstract. We prove a six-parameter identity whose terms have the form
xaT(kx, lx)T(k2 , l2), where T(k, I) = Y,-x xk"2+ln . This identity is then
used to give a new proof of the familiar Ramanujan identity H(x)G(x11) -
x2G(x)H{xu) = 1, where G(x) = Tl^oK1 " *5"+1)(l - x5"+4)]-' and
H(x) = n^oK1 - *5"+2)(l - x5n+3)]-' . Two other identities, called "bal-
anced Q2 identities", are also established through its use.
1. INTRODUCTION
It is our purpose in this paper to prove a fundamental identity, whose terms
are a certain type of modular form, and then to demonstrate its use in proving
other identities. This general identity, its proof, and the three applications of
the identity that are made here were discovered by a mixture of theoretical
studies and computer analyses of particular identities. This work is different
from that presented in [2-4], where the identity to be proved was first discovered
by a computer search and then was proved by a computer-assisted, theoretical
argument.
We will use the two single-variable T-functions 7b and Tx (cf. [4, equation
(2)]), defined byOO OO
T(k, I) = T0(k,l) dáfYíxkn2+ln = Y[(l -x2kn)(l + x2k"-k+,)(l + x2kn~k~l)
— oo n=l
andOO OO
Tx(k,l) dàiY/(-l)nxk"2+ln = Y[(l -x2kn)(l -x2k"-k+l)(l -x2k"-k-').
-oo «=1
We call an identity a " T2 identity" if each of its terms has the form
xaTtl(kx,lx)T(2(k2,l2),
where ex and e2 are 0 or 1. We also say that a T2 identity is "balanced" if
the first component pair (kx ,k2) in each of its terms is the same.
Received by the editor July 27, 1992.
1991 Mathematics Subject Classification. Primary 05A19.Key words and phrases. Triple and quintuple product, modular identity, balanced T2 and Q1
identity.
© 1993 American Mathematical Society
0025-5718/93 $1.00+ $.25 per page
83
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84 RICHARD BLECKSMITH, JOHN BRILLHART, AND IRVING GERST
In §2 we will prove the fundamental identity (Theorem 1). It will then be
used in §3 to give a new proof of a familiar identity of Ramanujan (Theorem 2)
and in §4 to provide proofs for two interesting identities, called "balanced Q2
identities" (Theorems 3 and 4). In all of these proofs the fundamental identity
is specialized to produce a small family of identities by assigning sets of values
to its parameters. The identity in question is then established by showing it to
be a linear combination of the identities in this family.
These three proofs illustrate a tentative proof method of three steps: ( 1 ) If
possible, transform the given identity into a T2 identity; (2) Balance this iden-
tity using the expansion formula in [4, p. 779]; (3) Determine if and how the
fundamental formula can be used to generate a small family of identities in
terms of which the given identity can be expressed as a linear combination.
2. The fundamental identity
The following general identity, which is of interest in its own right, is also
important in generating special sets of identities from which proofs of other
identities can be made.
In working with T-functions, it is often important to re-index their sums.
This can be accomplished by simple transformation rules, which are also useful
in putting T in "reduced" form. (The function Te (k, I) is in "reduced form"
when 0<l<k (cf. [4, p. 780]).) Then, for e e {0, 1}, there is
the negative rule [4,(13)]:
(2.1) Te(k,-l) = T((k,l),
the single-step formula [4, (14)]:
(2.2) xaT((k, I) = (-l)exa-('-k)T€(k,2k-l),
and
the general transformation formula: If / = 2kq + r, where q e Z , then
(2.3) xaT((k,l) = (-iy"xa-('2k-i'rTf(k,r).
Proof of (2.3). We have thatOO
xaTt(k, I) = xtt^(-l)(nxknl+l2k<i+r)n
— OO
OO
— x<* y/ \y[n-q)xk(n-q)2M2kq+r)(n-q)
— oo
= (-l)(<lx"-q2k-'irT((k,r). D
Note, when we use (2.3) to put T(k, I) numerically into reduced form, we
take r to be in the interval -k < r < k, and then use (2.1) if necessary.
Throughout the rest of this paper we will give all T-functions with numerical
arguments in reduced form.
Theorem 1. Suppose that m, u, v e Z + , e, f e Q, and k e Q+ , where
uv < 2m . Then
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A FUNDAMENTAL MODULAR IDENTITY AND SOME APPLICATIONS 85
(2.4) J2 xa"T(kx,lXn)T(k2,l2n)= £ xa»T(kx,l\n)T(k2,r2n),n€Rm n€R'm
where
2vk(2.5) a„ =-n +2en, kx = uk, k2 = (2m - uv)vk,
m
, 2uvk .hn =-n + ue + f,
m
(2.6)
■2vkI l2n = (2m-uv)(—n + e) -vf,
v m )
2uvkl\„ =-n + ue - f,'" m
l'2n = (2m - uv)(-n + ej +vf,<2vk
m
and Rm and R'm are any complete residue systems (mod m).
Proof. The proof rests on the following generalization of the expansion for-
mula in [4, p. 779]. (The notation in [4] is changed here to suit the present
proof. Note that the proof presented in [4] remains valid when we allow the
parameters k > 0 and / in T(k, I) = J^-^ xk"2+ln to be rational.)
Let a, b € Z, p e Z+, (kx , Àx), (k2 , X2) e Q+ x Q, and ex, e2 e {0, 1}.If the separability condition
(2.7) KXb = K2a(p - ab)
is satisfied, then
Tei(Kx,Àx)Tei(K2,À2)
(2.8) = ^{-^"x^^Ts^J^nWs^J^n)),n£Rf,
where
kx—KX+ K2a2, k2 = K2ß(p - ab),
lx(n) = ax - A2a - 2tc2an , l2(n) = (p.- ab)(2K2n + a2) + AXb,
¿i = (ei + é2#) (mod 2), 62 — (exb + e2(p - ab)) (mod 2),
and Ru is a complete residue system (mod p ).
In (2.8) substitute
(2m - uv)uk vkK\ =-»-— , k2 = —,
(2-9) if ; .v ; M=-f, h = e,
ex = u mod 2, e2 = 1,
and use the parameters a = u, b = v , and p = 2m . Note that (2.7) is satisfied,
since by (2.9)
kxv = K2u(p - uv).
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86 RICHARD BLECKSMITH, JOHN BRILLHART. AND IRVING GERST
We then have that
, -, (2m - uv)uk vku2 ,kx =kx+ K2aL =---+ -z— = uk,
2m 2m
(2.10) k2 = K2p(p-ab) = (2m-uv)vk,
ôx = (ex + e2u) mod 2 = 0,
S2 = (exv + e2(2m - uv)) mod 2-v(ex+ e2u) mod 2 = 0.
Since T = To, the expansion is then
(2.11) T(l(Kx,AX)Te2(K2,A2)= J2 (-irnxa(n)T(kx,lx(n))T(k2,l2(n)),
neRim
where (using -/i(zz) by (2.1))
a(n) = K2n2 + l2n = -—n2 + en,
(2.12) /,(«) = 2K2an + À2a - ax =-n + ue + f,m
/ v k \l2(n) = (p - ab)(2K2n + X2) + Xxb = (2m - uv)(—n + e) - vf.
\ m )
Now separate the sum on the right side of (2.11 ) into the difference of two sums,
Tu(kx , Xx)T(1(k2 , k2) =S+-S-,
where
S+ = 53 xa^T(kx , lx(2n)) Tik2, l2i2n)) and
(2.13) "!!;S- = 51 xn{2n+l)T(kx,lx(2n+ l))T(k2,l2(2n+ 1)).
neRm
Next, consider the expansion of the product
Ttl(Ki,-Ai)Te2(K2,A2)=S'+-SL
= 53 x"^T(kx,l[(2n))T(k2,l2(2n))neR'„,
53 x^2n+]^T(kx, l[(2n + 1)) T(k2,l'2(2n + 1)),n£R'„
where /{ and 1'2 are obtained from lx and l2 by replacing / by -/ in (2.12).
Since rf|(zci, ax) = T(1(kx , -ax) , we have that
(2.14) S+-S- =5;-51.
Now repeat the preceding development but with ex — e2 = 0 in (2.8). Since
all the terms of the resulting expansion are the same as before, except that they
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A FUNDAMENTAL MODULAR IDENTITY AND SOME APPLICATIONS 87
are now all positive, it follows that
T0(Kl,Al)To(K2,h) = S+ + S-
and
To(kx,-ax)T0(k2,a2)=S'+ + S'_,
so
(2.15) 5++ 5_ = 5;+51.
It then follows from (2.14) to (2.15) that 5+ = 5; and 5_ = 51. On the onehand, from (2.13) we have
5+= 53 xa"T(kx,lXn)T(k2,l2n),n€Rm
where from (2.12) we obtain
,_ , 2vk 7an = a(2n) =-n + 2en,
m, . ._ . 2uvk .h„ = h(2n) =-n + ue + f,
mjlvk
m
which is the left-hand side of (2.4). Similarly S'+ is the right-hand side of (2.4),so (2.4) is proved. D
¿2« = h(2n) = (2m - uv)(-n + e) - vf,\ m )
Remarks. 1. In (2.4) we can permit a„ to be negative, since negative powers of
x can be removed from the equation by multiplying through by an appropriate
power of x.
2. We do not obtain another identity from the equation 5_ = 51, since
this identity arises from (2.4) by replacing "e" by "e + ^ " in (2.5) and (2.6),
and multiplying the resulting identity by " xe+(vkl2m) ".
3. A NEW PROOF OF A RAMANUJAN IDENTITY
In this section and the next we will prove three identities using Theorem 1.
In each of these proofs a small family of identities is derived from (2.4) by
giving certain sets of values to its parameters. The identity in question is then
verified by showing it is a particular linear combination of these identities.
The first identity we will consider is a familiar result of Ramanujan.
gW = n(1_x5,+1)1(1_x5n+4) *«à H(x) = H 5n+2l){l_x5n+3),
Theorem 2 (Ramanujan [7, 9, 10, 1,6, 8]). If
1
n=0v" "' n=0
then
(3.1) H(x)G(xn)-x2G(x)H(xn)= 1.
Proof. The proof is in three steps.
(i) We begin by transforming (3.1) into a T2 identity. For simplicity, write
the infinite products in (3.1) in the abbreviated notation
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Page 6
88 RICHARD BLECKSMITH, JOHN BRILLHART, AND IRVING GERST
(ri, r) d^ n ;i — jc")n=l
., r, (mod m)
and then clear the fractions, which gives the identity
(±l)5(±22)ss - x2(±2)5(±l 1)55 = (±1, ±2)s (±11, ±22)55.
Multiplying through by (0)5 (0)55 and rewriting, we obtain the identity
(0, ±22)55(0, ±1)5 -x2(0, ±11)55 (0, ±2)5(3.2)
(0),(0)„=(0, ±11)33(0, ±1)3.
Identifying each factor in (3.2) as a certain Tx(k, /), we obtain the desired T2identity, viz.
(3.3) r,(f, l±)Tx(\,\)-x2Tx(%,^)Tx(\,{) = T,(f , if)7i(f, \).
(ii) We next use the expansion formula (cf. [4, p. 779]) (here we use the
original notation) on each of the three terms in (3.3) to convert it into a bal-
anced T2 identity. Program Forward [4, p. 789] indicates that both (kx, k2) =
(■y , f ) and (kx, k2) = (",§) can be expanded to the same pair (Kx, K2) =
(30, 330). We therefore expand the two terms on the left side of (3.3) intosums that are balanced at (30, 330), using the parameters [a, b, m] = [1, 1, 12]
(cf. [4, p. 780 ]). This gives respectively the two groups of twelve terms in
Table 1. Next, the term on the right of (3.3) is expanded using the parameters
[3, 3, 20], giving the twenty terms listed in Table 2. (Note that the form of each
term in all these expansions is xa'T0(30, lXi)T0(330, l2i), while in Tables 1 and2 we have indicated only the signs and the values of a¡, lXi, l2i.)
Table 1. Expansions using [1, 1, 12]
Group 1 terms Group 2 terms
7i(f ,%)Txi\,\), [1,1,12]
1234
56789101112
sign
+
++
++
+
0147
13
18273446
557077
hi
491
14
619112416292126
hi
22*337788*
132*143187198*242*253297308*
5 1x'Tx(^,^)Tx(^,^), [1, 1,12]
234
5
67891011
12
sign
+
+
+
+
24
5111322263444
516773
162111266
291
244
199
14
hi
22337788
132143187198242253297308
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Page 7
A FUNDAMENTAL MODULAR IDENTITY AND SOME APPLICATIONS
Table 2. Expansion of Tx(^ , ^)TX(\ , \) using [3, 3, 20]
Group 3 positive terms Group 3 negative terms
89
1234
567
89
10
057
12232437466277
/,
42214202
2810168
26
hi
22*44
88*110176154220242*286308*
12
34
567
8910
12
1115162933486279
/,
135
2923111971
2517
hi
11
5577
121143187209253275319
Table 3. A¡(x) and B¡(x)
Ai(x)=xaiT(30,lXi)T(330,l2i) Bi(x)=xaiT(30,lXi)T(330,l2i)
1234
567
8910II1213141516
hi hi
oi34
1014152125283247
50616678
135
211129231129
1197
119259
17
1155337777
121143143187187209253253275297319
1234
567
89101112
131415
16
hi
01
34
1011
17
2223263643
54616972
9161
222620192
2811104
298
21
14
33227744
88110143176154187220242253286297308
Equation (3.3) now becomes
(3.4) Group 1 terms - Group 2 terms = Group 3 terms.
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Page 8
90 RICHARD BLECKSMITH, JOHN BRILLHART, AND IRVING GERST
Four terms in Group 1 cancel across this equation with four positive terms in
Group 3 and two terms in Group 1 cancel the two negative terms in Group 2.
(These are marked with an asterisk in Tables 1 and 2.) This leaves 16 terms on
each side of (3.4). Transposing the negative terms that remain in (3.4) to the
other side of the equation and dividing out the factor x in all the terms, we
obtain the desired balanced T2 identity:
(3.5)
16 16
53^w = 53ß,wi=i r=l
where the A,(x) and B¡(x) are listed in Table 3. It is this equation we must
prove to establish (3.1).(iii) We now obtain three identities, given in Table 4 below, by choosing
three sets of values for the parameters m , k , u, v , e , and / in (2.4). The
first two identities have m = 6, £ = 30, u = v = 1, and (e, f) = (|, -y)
and (|, "), respectively. The third identity has m = 10, k = 10, u — v = 3,
and (e,f) = (\,xi).
Table 4. The three identities with kx = 30 and k2 = 330
Identity 1: m = 6, £ = 30, u = v = 1, e = \, /=-T
03
17
265467
212
8221828
4466
154176264286
137
101315
03
17
265469
91
1911
2921
3377
143187253297
Identity 2: m = 6, /: = 30, u = v 9 f.3> / '33T
34
891315
0 21
182247
63
291
3377
143187253297
9*
14
01
19205158
12
222
2818
6644
176154264286
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A FUNDAMENTAL MODULAR IDENTITY AND SOME APPLICATIONS
Identity 3: w = 10, Â: = 10, u = v = 3, e = § , / = T
91
1
25
67
101112
1416
01
1014
15283247
6178
/,
135
2923111971
2517
h
11
5577
121143187209253275319
11
12*
16
1011
17
2636436772
/,
21626208
22104
2814
h
44
2288
110154176220242286308
Now form the following linear combination of the above three identities,
(3.6) Identity 1 + jc3 x Identity 2 + Identity 3,
which yields equation (3.5) when six terms on the left-hand side of Identity 1
are cancelled with six terms on the right-hand side of Identities 2 and 3. (The
terms that cancel are indicated in the tables by an asterisk. The term numbers
in the # columns for the left and right sides in Table 4 correspond respectively
to those in the "z" columns in Table 3. Note that the a-values in Identity 2
must be increased by 3 because of the multiplier x3.) □
4. TWO BALANCED Q2 IDENTITIES
Within the set of balanced T2 identities there is a special and interesting
subset—the balanced Q2 identities. (Here Q stands for the usual quintuple
product
Q(m,k)def }(1 -xn) = YJxm(i"2+n)l2(x-
n€S -oo
3kn ,lkn+k)•
0, ±k , ±(m - 2k), ±(m - k), m (mod 2m)}where 5 = {n e Z+ :
The two identities of this kind which we will prove in this section are tri-
nomials of similar form. They have similar proofs as well, but are of quite
different types, as we will show in another paper. Because these identities are
already balanced, there is no need to balance them as we did in the proof of
Theorem 2. At present, these are two of the three balanced Q2 identities we
know.
(a) The first of these identities was originally proved as part of a proof in
[4, equation (16)]. To our knowledge, this is the first example of a balanced Q2
identity to appear in the literature.
Theorem 3. We have that
(4.1) (2(8, 3)0(56, 7) + x3Q(S, 1)0(56, 21) = 0(8, 2)0(56, 14).
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92 RICHARD BLECKSMITH, JOHN BRILLHART, AND IRVING GERST
Proof. Since (4.1) is a Q2 identity (balanced at (8, 56)), we can routinely
rewrite it as a T2 identity (balanced at (12, 84)) by first transforming each Qin (4.1) into T terms by the following formula [4, equation (24)]:
(4.2) Q(m,k) = T(-j-, y - 3fcj - xkT[— , - + 3k).
Therefore, using (2.1) and (2.2), we have
ß(8,3) = 77(12,5)-x277(12, 11), ß(8, 1) = 77(12, l)-jrT(12, 7),
and
ß(8, 2) = 77(12, 2)-x2r( 12, 10).
Replacing x by x1 in these three results, we obtain
0(56,21) = 77(84, 35) - x1477(84, 77),
ß(56, 7) = 77(84, 7)-x777(84, 49),
and
ß(56, 14) = 77(84, 14)-x14r(84, 70).
Thus, the first term in (4.1) becomes
ß(8, 3)0(56, 7)= [77(12, 5)-x277(12, 11)][77(84, 7) - x777(84, 49)]
(4.3) =5(0, 5, 7)-5(2, 11, 7) - 5(7, 5, 49)
+ 5(9, 11,49),
where for simplicity we have written
5(a,/1,/2) = x«r(12,/1)77(84,/2).
If we expand the second and third terms in (4.1) in the same way, we obtain
x3ß(8, l)ß(56, 21) =5(3, 1, 35)-5(4, 7, 35)-5(17, 1, 77)+ 5(18, 7, 77)
and
ß(8, 2)ß(56, 14) = 5(0, 2, 14)-5(2, 10, 14)-5(14,2, 70)+5(16, 10, 70).
With the help of these three results, identity (4.1) becomes the balanced 772
identity
5(0, 5, 7)+ 5(2, 10, 14)+ 5(3, 1,35)+ 5(9, 11,49)
+ 5(14,2,70) + 5(18,7,77)
( ' ' =5(0,2, 14)+ 5(2, ll,7) + 5(4,7,35) + 5(7,5,49)
+ 5(16, 10, 70)+ 5(17, 1, 77).
To prove (4.4), we use the subsidiary identity derived from Theorem 1 by setting
m = 4, k = 12, and u = v = 1, viz.
3
535(6«2 + 2e«, 6n + e + f, 7(6«+ <?)-/)
(4.5) "=° 3
= ¿5(6«2 + 2é*«, 6n + e-f, 7(6« + <?)+ /).n=0
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A FUNDAMENTAL MODULAR IDENTITY AND SOME APPLICATIONS 93
If we put (e, f) = (\, ]) and (e, f) = (-\, y) into (4.5), we obtain respec-tively the two identities
5(0,5,7) + 5(3, l,35) + 5(9, 11, 49)+5(18, 7, 77)[ ' ' =5(0,2, 14) + 5(3,8,28)+5(9,4,56) + 5(16, 10,70)
and
5(0, 10, 14)+5(l,8,28) + 5(7,4,56) + 5(12,2,70)
= 5(0, ll,7) + 5(2,7,35) + 5(5,5,49) + 5(15, 1,77).
Then the linear combination (4.6) + x2 x (4.7) of these equations produces
(4.4) when the common sum 5(3, 8, 28) +5(9, 4, 56) is cancelled from itstwo sides. (Note that multiplying an 5 by x" increases the first argument of
5 by n .) G
Remark. The proof of (4.1) provides a third proof of identity (9) in [4].
(b) In the summer of 1990 we sent identity (4.1) to Oliver Atkin. Within24 hours he sent us identity (4.8) below, another trinomial identity with a form
similar to that of (4.1). An impressive performance!
Theorem 4 [Atkin]. We have that
(4.8) ß(14,3)ß(70,5) + x3ß(14, l)ß(70, 25) = ß(14, 5)ß(70, 15).
Proof. As in the proof of (4.1), we first use (4.2) to transform (4.8) into a
T2 equation, balanced at (21, 105). We find that
ß(14,3) = 7(21,2)-x37(21, 16), ß(14, 1) = 77(21, 4) -xT(2l, 10),
and
ß(14,5) = 7(21,8)-x47(21,20),
and, replacing x by x5 in these results, we obtain the equations
ß(70, 15) = 77(105, 10)-x157(105, 80),
ß(70, 5) = 7(105, 20)-x577(105, 50),
and
ß(70,25) = 7(105, 40)-x207(105, 100).
Thus, the three terms in (4.8) transform into
ß(14, 3)ß(70, 5) = 5(0, 2, 20)-5(3, 16,20)-5(5,2, 50) + 5(8, 16, 50),(4.9)
(4.10)
and
(4.11)
x3Q(l4, l)ß(70, 25) = 5(3, 4, 40)-5(4, 10,40)-5(23,4, 100) + 5(24, 10, 100),
ß(14, 5)0(70, 15) = 5(0, 8, 10)-5(4, 20, 10)-5(15, 8, 80) + 5(19,20, 80),
where we have written
5(a,/,,/2) = x"7(21,/1)7(105,/2).
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94 RICHARD BLECKSMITH, JOHN BRILLHART, AND IRVING GERST
Thus, equation (4.8) becomes the balanced 72 equation
5(0, 2, 20) + 5(3, 4, 40) + 5(4, 20, 10) + 5(8, 16, 50)
+ 5(15, 8, 80)+ 5(24, 10, 100)(" ' =5(0, 8, 10)+ 5(3, 16,20) + 5(4, 10, 40)+ 5(5, 2, 50)
+ 5(19,20, 80)+ 5(23, 4, 100).
Again we derive a subsidiary identity from Theorem 1 by setting m = 3,k — 21, and u = v = I, viz.
2
535(14«2 + 2e«, 14n + e + f, 5(14« + e) - f)
(4.13) "=0
535(14«2 + 2e«, I4n + e-f, 5(\4n + e) + f).n=0
Putting (e, f) = (3, -5), (6, -10), (5, 15) into (4.13) gives respectively thefamily of three identities
(4.14)
(4.15)
and
(4.16)
5(0, 2, 20)+5(8, 16, 50) + 5(20, 12, 90)
= 5(0,8, 10) + 5(8,6,60) + 5(19,20,80),
5(0, 4, 40)+ 5(2, 18, 30)+ 5(21, 10, 100)
= 5(0, 16, 20) + 5(2, 2, 50) + 5(17, 12, 90),
5(0,20, 10) + 5(4,6,60) + 5(ll, 8, 80)
= 5(0, 10,40)+5(1, 18, 30) + 5(19,4, 100).
The linear combination (4.14) +x3 x (4.15) +x4 x (4.16) of these equations
gives an identity which becomes (4.12) when the common sum 5(5, 18, 30) +
5(8, 6, 60) + 5(20, 12, 90) is cancelled from its two sides. □
Acknowledgment
We would like to thank Greg Manning for his help in developing the algo-
rithms that were used to find the proofs in this paper.
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Department of Mathematical Sciences, Northern Illinois University, DeKalb, Illi-
nois 60115E-mail address : [email protected]
Department of Mathematics, University of Arizona, Tucson, Arizona 85721
E-mail address: [email protected]
Department of Applied Mathematics and Statistics, SUNY at Stony Brook, Stony
Brook, New York 11794
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