This module provided some upper and lower bounds for the parameters of codes.
AUTHORS:
Let \(F\) be a finite field (we denote the finite field with \(q\) elements by \(\GF{q}\)). A subset \(C\) of \(V=F^n\) is called a code of length \(n\). A subspace of \(V\) (with the standard basis) is called a linear code of length \(n\). If its dimension is denoted \(k\) then we typically store a basis of \(C\) as a \(k\times n\) matrix (the rows are the basis vectors). If \(F=\GF{2}\) then \(C\) is called a binary code. If \(F\) has \(q\) elements then \(C\) is called a \(q\)-ary code. The elements of a code \(C\) are called codewords. The information rate of \(C\) is
where \(\vert C\vert\) denotes the number of elements of \(C\). If \({\bf v}=(v_1,v_2,...,v_n)\), \({\bf w}=(w_1,w_2,...,w_n)\) are vectors in \(V=F^n\) then we define
to be the Hamming distance between \({\bf v}\) and \({\bf w}\). The function \(d:V\times V\rightarrow \Bold{N}\) is called the Hamming metric. The weight of a vector (in the Hamming metric) is \(d({\bf v},{\bf 0})\). The minimum distance of a linear code is the smallest non-zero weight of a codeword in \(C\). The relatively minimum distance is denoted
A linear code with length \(n\), dimension \(k\), and minimum distance \(d\) is called an \([n,k,d]_q\)-code and \(n,k,d\) are called its parameters. A (not necessarily linear) code \(C\) with length \(n\), size \(M=|C|\), and minimum distance \(d\) is called an \((n,M,d)_q\)-code (using parentheses instead of square brackets). Of course, \(k=\log_q(M)\) for linear codes.
What is the “best” code of a given length? Let \(F\) be a finite field with \(q\) elements. Let \(A_q(n,d)\) denote the largest \(M\) such that there exists a \((n,M,d)\) code in \(F^n\). Let \(B_q(n,d)\) (also denoted \(A^{lin}_q(n,d)\)) denote the largest \(k\) such that there exists a \([n,k,d]\) code in \(F^n\). (Of course, \(A_q(n,d)\geq B_q(n,d)\).) Determining \(A_q(n,d)\) and \(B_q(n,d)\) is one of the main problems in the theory of error-correcting codes.
These quantities related to solving a generalization of the childhood game of “20 questions”.
GAME: Player 1 secretly chooses a number from \(1\) to \(M\) (\(M\) is large but fixed). Player 2 asks a series of “yes/no questions” in an attempt to determine that number. Player 1 may lie at most \(e\) times (\(e\geq 0\) is fixed). What is the minimum number of “yes/no questions” Player 2 must ask to (always) be able to correctly determine the number Player 1 chose?
If feedback is not allowed (the only situation considered here), call this minimum number \(g(M,e)\).
Lemma: For fixed \(e\) and \(M\), \(g(M,e)\) is the smallest \(n\) such that \(A_2(n,2e+1)\geq M\).
Thus, solving the solving a generalization of the game of “20 questions” is equivalent to determining \(A_2(n,d)\)! Using Sage, you can determine the best known estimates for this number in 2 ways:
which connects to the website http://www.codetables.de by Markus Grassl;
and best_known_linear_code(n, k, F).
The output of best_known_linear_code(), best_known_linear_code_www(), or dimension_upper_bound() would give only special solutions to the GAME because the bounds are applicable to only linear codes. The output of codesize_upper_bound() would give the best possible solution, that may belong to a linear or nonlinear code.
This module implements:
PROBLEM: In this module we shall typically either (a) seek bounds on k, given n, d, q, (b) seek bounds on R, delta, q (assuming n is “infinity”).
TODO:
REFERENCES:
This computes the minimum value of the upper bound using the methods of Singleton, Hamming, Plotkin, and Elias.
If algorithm=”gap” then this returns the best known upper bound \(A(n,d)=A_q(n,d)\) for the size of a code of length n, minimum distance d over a field of size q. The function first checks for trivial cases (like d=1 or n=d), and if the value is in the built-in table. Then it calculates the minimum value of the upper bound using the algorithms of Singleton, Hamming, Johnson, Plotkin and Elias. If the code is binary, \(A(n, 2\ell-1) = A(n+1,2\ell)\), so the function takes the minimum of the values obtained from all algorithms for the parameters \((n, 2\ell-1)\) and \((n+1, 2\ell)\). This wraps GUAVA’s (i.e. GAP’s package Guava) UpperBound( n, d, q ).
If algorithm=”LP” then this returns the Delsarte (a.k.a. Linear Programming) upper bound.
EXAMPLES:
sage: codesize_upper_bound(10,3,2)
93
sage: codesize_upper_bound(24,8,2,algorithm="LP")
4096
sage: codesize_upper_bound(10,3,2,algorithm="gap") # optional - gap_packages (Guava package)
85
sage: codesize_upper_bound(11,3,4,algorithm=None)
123361
sage: codesize_upper_bound(11,3,4,algorithm="gap") # optional - gap_packages (Guava package)
123361
sage: codesize_upper_bound(11,3,4,algorithm="LP")
109226
Returns an upper bound \(B(n,d) = B_q(n,d)\) for the dimension of a linear code of length n, minimum distance d over a field of size q. Parameter “algorithm” has the same meaning as in codesize_upper_bound()
EXAMPLES:
sage: dimension_upper_bound(10,3,2)
6
sage: dimension_upper_bound(30,15,4)
13
sage: dimension_upper_bound(30,15,4,algorithm="LP")
12
Computes the asymptotic Elias bound for the information rate, provided \(0 < \delta < 1-1/q\).
EXAMPLES:
sage: elias_bound_asymp(1/4,2)
0.39912396330...
Returns the Elias upper bound for number of elements in the largest code of minimum distance d in \(\GF{q}^n\). Wraps GAP’s UpperBoundElias.
EXAMPLES:
sage: elias_upper_bound(10,2,3)
232
sage: elias_upper_bound(10,2,3,algorithm="gap") # optional - gap_packages (Guava package)
232
Computes the entropy at \(x\) on the \(q\)-ary symmetric channel.
INPUT:
EXAMPLES:
sage: entropy(0, 2)
0
sage: entropy(1/5,4)
1/5*log(3)/log(4) - 4/5*log(4/5)/log(4) - 1/5*log(1/5)/log(4)
sage: entropy(1, 3)
log(2)/log(3)
Check that values not within the limits are properly handled:
sage: entropy(1.1, 2)
Traceback (most recent call last):
...
ValueError: The entropy function is defined only for x in the interval [0, 1]
sage: entropy(1, 1)
Traceback (most recent call last):
...
ValueError: The value q must be an integer greater than 1
Returns lower bound for number of elements in the largest code of minimum distance d in \(\GF{q}^n\).
EXAMPLES:
sage: gilbert_lower_bound(10,2,3)
128/7
Returns the Griesmer upper bound for number of elements in the largest code of minimum distance d in \(\GF{q}^n\). Wraps GAP’s UpperBoundGriesmer.
EXAMPLES:
sage: griesmer_upper_bound(10,2,3)
128
sage: griesmer_upper_bound(10,2,3,algorithm="gap") # optional - gap_packages (Guava package)
128
Computes the asymptotic GV bound for the information rate, R.
EXAMPLES:
sage: RDF(gv_bound_asymp(1/4,2))
0.188721875541
sage: f = lambda x: gv_bound_asymp(x,2)
sage: plot(f,0,1)
GV lower bound for information rate of a q-ary code of length n minimum distance delta*n
EXAMPLES:
sage: RDF(gv_info_rate(100,1/4,3))
0.367049926083
Computes the asymptotic Hamming bound for the information rate.
EXAMPLES:
sage: RDF(hamming_bound_asymp(1/4,2))
0.4564355568
sage: f = lambda x: hamming_bound_asymp(x,2)
sage: plot(f,0,1)
Returns the Hamming upper bound for number of elements in the largest code of minimum distance d in \(\GF{q}^n\). Wraps GAP’s UpperBoundHamming.
The Hamming bound (also known as the sphere packing bound) returns an upper bound on the size of a code of length n, minimum distance d, over a field of size q. The Hamming bound is obtained by dividing the contents of the entire space \(\GF{q}^n\) by the contents of a ball with radius floor((d-1)/2). As all these balls are disjoint, they can never contain more than the whole vector space.
where M is the maximum number of codewords and \(V(n,e)\) is equal to the contents of a ball of radius e. This bound is useful for small values of d. Codes for which equality holds are called perfect.
EXAMPLES:
sage: hamming_upper_bound(10,2,3)
93
Computes the first asymptotic McEliese-Rumsey-Rodemich-Welsh bound for the information rate, provided \(0 < \delta < 1-1/q\).
EXAMPLES:
sage: mrrw1_bound_asymp(1/4,2)
0.354578902665
Computes the asymptotic Plotkin bound for the information rate, provided \(0 < \delta < 1-1/q\).
EXAMPLES:
sage: plotkin_bound_asymp(1/4,2)
1/2
Returns Plotkin upper bound for number of elements in the largest code of minimum distance d in \(\GF{q}^n\).
The algorithm=”gap” option wraps Guava’s UpperBoundPlotkin.
EXAMPLES:
sage: plotkin_upper_bound(10,2,3)
192
sage: plotkin_upper_bound(10,2,3,algorithm="gap") # optional - gap_packages (Guava package)
192
Computes the asymptotic Singleton bound for the information rate.
EXAMPLES:
sage: singleton_bound_asymp(1/4,2)
3/4
sage: f = lambda x: singleton_bound_asymp(x,2)
sage: plot(f,0,1)
Returns the Singleton upper bound for number of elements in the largest code of minimum distance d in \(\GF{q}^n\). Wraps GAP’s UpperBoundSingleton.
This bound is based on the shortening of codes. By shortening an \((n, M, d)\) code d-1 times, an \((n-d+1,M,1)\) code results, with \(M \leq q^n-d+1\). Thus
Codes that meet this bound are called maximum distance separable (MDS).
EXAMPLES:
sage: singleton_upper_bound(10,2,3)
256
Returns number of elements in a Hamming ball of radius r in \(\GF{q}^n\). Agrees with Guava’s SphereContent(n,r,GF(q)).
EXAMPLES:
sage: volume_hamming(10,2,3)
176