In number theory, a congruent number is a positive integer that is the area of a right triangle with three rational number sides.[1][2] A more general definition includes all positive rational numbers with this property.[3]
![](http://upload.wikimedia.org/wikipedia/commons/thumb/d/da/Rtriangle-mathsinegypt.svg/220px-Rtriangle-mathsinegypt.svg.png)
The sequence of (integer) congruent numbers starts with
- 5, 6, 7, 13, 14, 15, 20, 21, 22, 23, 24, 28, 29, 30, 31, 34, 37, 38, 39, 41, 45, 46, 47, 52, 53, 54, 55, 56, 60, 61, 62, 63, 65, 69, 70, 71, 77, 78, 79, 80, 84, 85, 86, 87, 88, 92, 93, 94, 95, 96, 101, 102, 103, 109, 110, 111, 112, 116, 117, 118, 119, 120, ... (sequence A003273 in the OEIS)
n | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
---|---|---|---|---|---|---|---|---|
— | — | — | — | C | C | C | — | |
n | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 |
— | — | — | — | C | C | C | — | |
n | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 |
— | — | — | S | C | C | C | S | |
n | 25 | 26 | 27 | 28 | 29 | 30 | 31 | 32 |
— | — | — | S | C | C | C | — | |
n | 33 | 34 | 35 | 36 | 37 | 38 | 39 | 40 |
— | C | — | — | C | C | C | — | |
n | 41 | 42 | 43 | 44 | 45 | 46 | 47 | 48 |
C | — | — | — | S | C | C | — | |
n | 49 | 50 | 51 | 52 | 53 | 54 | 55 | 56 |
— | — | — | S | C | S | C | S | |
n | 57 | 58 | 59 | 60 | 61 | 62 | 63 | 64 |
— | — | — | S | C | C | S | — | |
n | 65 | 66 | 67 | 68 | 69 | 70 | 71 | 72 |
C | — | — | — | C | C | C | — | |
n | 73 | 74 | 75 | 76 | 77 | 78 | 79 | 80 |
— | — | — | — | C | C | C | S | |
n | 81 | 82 | 83 | 84 | 85 | 86 | 87 | 88 |
— | — | — | S | C | C | C | S | |
n | 89 | 90 | 91 | 92 | 93 | 94 | 95 | 96 |
— | — | — | S | C | C | C | S | |
n | 97 | 98 | 99 | 100 | 101 | 102 | 103 | 104 |
— | — | — | — | C | C | C | — | |
n | 105 | 106 | 107 | 108 | 109 | 110 | 111 | 112 |
— | — | — | — | C | C | C | S | |
n | 113 | 114 | 115 | 116 | 117 | 118 | 119 | 120 |
— | — | — | S | S | C | C | S |
For example, 5 is a congruent number because it is the area of a (20/3, 3/2, 41/6) triangle. Similarly, 6 is a congruent number because it is the area of a (3,4,5) triangle. 3 and 4 are not congruent numbers.
If q is a congruent number then s2q is also a congruent number for any natural number s (just by multiplying each side of the triangle by s), and vice versa. This leads to the observation that whether a nonzero rational number q is a congruent number depends only on its residue in the group
where is the set of nonzero rational numbers.
Every residue class in this group contains exactly one square-free integer, and it is common, therefore, only to consider square-free positive integers, when speaking about congruent numbers.
Congruent number problem
editThe question of determining whether a given rational number is a congruent number is called the congruent number problem. This problem has not (as of 2019) been brought to a successful resolution. Tunnell's theorem provides an easily testable criterion for determining whether a number is congruent; but his result relies on the Birch and Swinnerton-Dyer conjecture, which is still unproven.
Fermat's right triangle theorem, named after Pierre de Fermat, states that no square number can be a congruent number. However, in the form that every congruum (the difference between consecutive elements in an arithmetic progression of three squares) is non-square, it was already known (without proof) to Fibonacci.[4] Every congruum is a congruent number, and every congruent number is a product of a congruum and the square of a rational number.[5] However, determining whether a number is a congruum is much easier than determining whether it is congruent, because there is a parameterized formula for congrua for which only finitely many parameter values need to be tested.[6]
Solutions
editn is a congruent number if and only if the system
,
has a solution where , and
are integers.[7]
Given a solution, the three numbers ,
, and
will be in an arithmetic progression with common difference
.
Furthermore, if there is one solution (where the right-hand sides are squares), then there are infinitely many: given any solution ,another solution
can be computed from[8]
For example, with , the equations are:
One solution is (so that
). Another solution is
With this new and
, the new right-hand sides are still both squares:
Using as above gives
Given , and
, one can obtain
, and
such that
, and
from
Then and
are the legs and hypotenuse of a right triangle with area
.
The above values produce
. The values
give
. Both of these right triangles have area
.
Relation to elliptic curves
editThe question of whether a given number is congruent turns out to be equivalent to the condition that a certain elliptic curve has positive rank.[3] An alternative approach to the idea is presented below (as can essentially also be found in the introduction to Tunnell's paper).
Suppose a, b, c are numbers (not necessarily positive or rational) which satisfy the following two equations:
Then set x = n(a + c)/b andy = 2n2(a + c)/b2.A calculation shows
and y is not 0 (if y = 0 then a = −c, so b = 0, but (1⁄2)ab = n is nonzero, a contradiction).
Conversely, if x and y are numbers which satisfy the above equation and y is not 0, seta = (x2 − n2)/y,b = 2nx/y, and c = (x2 + n2)/y. A calculation shows these three numberssatisfy the two equations for a, b, and c above.
These two correspondences between (a,b,c) and (x,y) are inverses of each other, sowe have a one-to-one correspondence between any solution of the two equations ina, b, and c and any solution of the equation in x and y with y nonzero. In particular,from the formulas in the two correspondences, for rational n we see that a, b, and c arerational if and only if the corresponding x and y are rational, and vice versa.(We also have that a, b, and c are all positive if and only if x and y are all positive;from the equation y2 = x3 − xn2 = x(x2 − n2)we see that if x and y are positive then x2 − n2 must be positive, so the formula fora above is positive.)
Thus a positive rational number n is congruent if and only if the equationy2 = x3 − n2x has a rational point with y not equal to 0.It can be shown (as an application of Dirichlet's theorem on primes in arithmetic progression)that the only torsion points on this elliptic curve are those with y equal to 0, hence theexistence of a rational point with y nonzero is equivalent to saying the elliptic curve has positive rank.
Another approach to solving is to start with integer value of n denoted as N and solve
where
Current progress
editFor example, it is known that for a prime number p, the following holds:[9]
- if p ≡ 3 (mod 8), then p is not a congruent number, but 2p is a congruent number.
- if p ≡ 5 (mod 8), then p is a congruent number.
- if p ≡ 7 (mod 8), then p and 2p are congruent numbers.
It is also known that in each of the congruence classes 5, 6, 7 (mod 8), for any given k there are infinitely many square-free congruent numbers with k prime factors.[10]
Notes
edit- ^ Weisstein, Eric W. "Congruent Number". MathWorld.
- ^ Guy, Richard K. (2004). Unsolved problems in number theory ([3rd ed.] ed.). New York: Springer. pp. 195–197. ISBN 0-387-20860-7. OCLC 54611248.
- ^ a b Koblitz, Neal (1993), Introduction to Elliptic Curves and Modular Forms, New York: Springer-Verlag, p. 3, ISBN 0-387-97966-2
- ^ Ore, Øystein (2012), Number Theory and Its History, Courier Dover Corporation, pp. 202–203, ISBN 978-0-486-13643-1.
- ^ Conrad, Keith (Fall 2008), "The congruent number problem" (PDF), Harvard College Mathematical Review, 2 (2): 58–73, archived from the original (PDF) on 2013-01-20.
- ^ Darling, David (2004), The Universal Book of Mathematics: From Abracadabra to Zeno's Paradoxes, John Wiley & Sons, p. 77, ISBN 978-0-471-66700-1.
- ^ Uspensky, J. V.; Heaslet, M. A. (1939). Elementary Number Theory. Vol. 2. McGraw Hill. p. 419.
- ^ Dickson, Leonard Eugene (1966). History of the Theory of Numbers. Vol. 2. Chelsea. pp. 468–469.
- ^ Paul Monsky (1990), "Mock Heegner Points and Congruent Numbers", Mathematische Zeitschrift, 204 (1): 45–67, doi:10.1007/BF02570859, S2CID 121911966
- ^ Tian, Ye (2014), "Congruent numbers and Heegner points", Cambridge Journal of Mathematics, 2 (1): 117–161, arXiv:1210.8231, doi:10.4310/CJM.2014.v2.n1.a4, MR 3272014, S2CID 55390076.
References
edit- Alter, Ronald (1980), "The Congruent Number Problem", American Mathematical Monthly, 87 (1), Mathematical Association of America: 43–45, doi:10.2307/2320381, JSTOR 2320381
- Chandrasekar, V. (1998), "The Congruent Number Problem" (PDF), Resonance, 3 (8): 33–45, doi:10.1007/BF02837344, S2CID 123495100
- Dickson, Leonard Eugene (2005), "Chapter XVI", History of the Theory of Numbers, Dover Books on Mathematics, vol. II: Diophantine Analysis, Dover Publications, ISBN 978-0-486-44233-4 – see, for a history of the problem.
- Guy, Richard (2004), Unsolved Problems in Number Theory, Problem Books in Mathematics (Book 1) (3rd ed.), Springer, ISBN 978-0-387-20860-2, Zbl 1058.11001 – Many references are given in it.
- Tunnell, Jerrold B. (1983), "A classical Diophantine problem and modular forms of weight 3/2", Inventiones Mathematicae, 72 (2): 323–334, Bibcode:1983InMat..72..323T, doi:10.1007/BF01389327, hdl:10338.dmlcz/137483
External links
edit- Weisstein, Eric W. "Congruent Number". MathWorld.
- A short discussion of the current state of the problem with many references can be found in Alice Silverberg's Open Questions in Arithmetic Algebraic Geometry (Postscript).
- A Trillion Triangles - mathematicians have resolved the first one trillion cases (conditional on the Birch and Swinnerton-Dyer conjecture).