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'hadamard', 'leslie', 'kron', 'block_diag', 'companion', 'helmert', 'hilbert', 'invhilbert', 'pascal', 'invpascal', 'dft']
#----------------------------------------------------------------------------- # matrix construction functions #-----------------------------------------------------------------------------
# # *Note*: tri{,u,l} is implemented in numpy, but an important bug was fixed in # 2.0.0.dev-1af2f3, the following tri{,u,l} definitions are here for backwards # compatibility.
""" Construct (N, M) matrix filled with ones at and below the k-th diagonal.
The matrix has A[i,j] == 1 for i <= j + k
Parameters ---------- N : int The size of the first dimension of the matrix. M : int or None, optional The size of the second dimension of the matrix. If `M` is None, `M = N` is assumed. k : int, optional Number of subdiagonal below which matrix is filled with ones. `k` = 0 is the main diagonal, `k` < 0 subdiagonal and `k` > 0 superdiagonal. dtype : dtype, optional Data type of the matrix.
Returns ------- tri : (N, M) ndarray Tri matrix.
Examples -------- >>> from scipy.linalg import tri >>> tri(3, 5, 2, dtype=int) array([[1, 1, 1, 0, 0], [1, 1, 1, 1, 0], [1, 1, 1, 1, 1]]) >>> tri(3, 5, -1, dtype=int) array([[0, 0, 0, 0, 0], [1, 0, 0, 0, 0], [1, 1, 0, 0, 0]])
""" if M is None: M = N if isinstance(M, string_types): #pearu: any objections to remove this feature? # As tri(N,'d') is equivalent to tri(N,dtype='d') dtype = M M = N m = np.greater_equal.outer(np.arange(k, N+k), np.arange(M)) if dtype is None: return m else: return m.astype(dtype)
""" Make a copy of a matrix with elements above the k-th diagonal zeroed.
Parameters ---------- m : array_like Matrix whose elements to return k : int, optional Diagonal above which to zero elements. `k` == 0 is the main diagonal, `k` < 0 subdiagonal and `k` > 0 superdiagonal.
Returns ------- tril : ndarray Return is the same shape and type as `m`.
Examples -------- >>> from scipy.linalg import tril >>> tril([[1,2,3],[4,5,6],[7,8,9],[10,11,12]], -1) array([[ 0, 0, 0], [ 4, 0, 0], [ 7, 8, 0], [10, 11, 12]])
""" m = np.asarray(m) out = tri(m.shape[0], m.shape[1], k=k, dtype=m.dtype.char) * m return out
""" Make a copy of a matrix with elements below the k-th diagonal zeroed.
Parameters ---------- m : array_like Matrix whose elements to return k : int, optional Diagonal below which to zero elements. `k` == 0 is the main diagonal, `k` < 0 subdiagonal and `k` > 0 superdiagonal.
Returns ------- triu : ndarray Return matrix with zeroed elements below the k-th diagonal and has same shape and type as `m`.
Examples -------- >>> from scipy.linalg import triu >>> triu([[1,2,3],[4,5,6],[7,8,9],[10,11,12]], -1) array([[ 1, 2, 3], [ 4, 5, 6], [ 0, 8, 9], [ 0, 0, 12]])
""" m = np.asarray(m) out = (1 - tri(m.shape[0], m.shape[1], k - 1, m.dtype.char)) * m return out
""" Construct a Toeplitz matrix.
The Toeplitz matrix has constant diagonals, with c as its first column and r as its first row. If r is not given, ``r == conjugate(c)`` is assumed.
Parameters ---------- c : array_like First column of the matrix. Whatever the actual shape of `c`, it will be converted to a 1-D array. r : array_like, optional First row of the matrix. If None, ``r = conjugate(c)`` is assumed; in this case, if c[0] is real, the result is a Hermitian matrix. r[0] is ignored; the first row of the returned matrix is ``[c[0], r[1:]]``. Whatever the actual shape of `r`, it will be converted to a 1-D array.
Returns ------- A : (len(c), len(r)) ndarray The Toeplitz matrix. Dtype is the same as ``(c[0] + r[0]).dtype``.
See Also -------- circulant : circulant matrix hankel : Hankel matrix solve_toeplitz : Solve a Toeplitz system.
Notes ----- The behavior when `c` or `r` is a scalar, or when `c` is complex and `r` is None, was changed in version 0.8.0. The behavior in previous versions was undocumented and is no longer supported.
Examples -------- >>> from scipy.linalg import toeplitz >>> toeplitz([1,2,3], [1,4,5,6]) array([[1, 4, 5, 6], [2, 1, 4, 5], [3, 2, 1, 4]]) >>> toeplitz([1.0, 2+3j, 4-1j]) array([[ 1.+0.j, 2.-3.j, 4.+1.j], [ 2.+3.j, 1.+0.j, 2.-3.j], [ 4.-1.j, 2.+3.j, 1.+0.j]])
""" c = np.asarray(c).ravel() if r is None: r = c.conjugate() else: r = np.asarray(r).ravel() # Form a 1D array containing a reversed c followed by r[1:] that could be # strided to give us toeplitz matrix. vals = np.concatenate((c[::-1], r[1:])) out_shp = len(c), len(r) n = vals.strides[0] return as_strided(vals[len(c)-1:], shape=out_shp, strides=(-n, n)).copy()
""" Construct a circulant matrix.
Parameters ---------- c : (N,) array_like 1-D array, the first column of the matrix.
Returns ------- A : (N, N) ndarray A circulant matrix whose first column is `c`.
See Also -------- toeplitz : Toeplitz matrix hankel : Hankel matrix solve_circulant : Solve a circulant system.
Notes ----- .. versionadded:: 0.8.0
Examples -------- >>> from scipy.linalg import circulant >>> circulant([1, 2, 3]) array([[1, 3, 2], [2, 1, 3], [3, 2, 1]])
""" c = np.asarray(c).ravel() # Form an extended array that could be strided to give circulant version c_ext = np.concatenate((c[::-1], c[:0:-1])) L = len(c) n = c_ext.strides[0] return as_strided(c_ext[L-1:], shape=(L, L), strides=(-n, n)).copy()
""" Construct a Hankel matrix.
The Hankel matrix has constant anti-diagonals, with `c` as its first column and `r` as its last row. If `r` is not given, then `r = zeros_like(c)` is assumed.
Parameters ---------- c : array_like First column of the matrix. Whatever the actual shape of `c`, it will be converted to a 1-D array. r : array_like, optional Last row of the matrix. If None, ``r = zeros_like(c)`` is assumed. r[0] is ignored; the last row of the returned matrix is ``[c[-1], r[1:]]``. Whatever the actual shape of `r`, it will be converted to a 1-D array.
Returns ------- A : (len(c), len(r)) ndarray The Hankel matrix. Dtype is the same as ``(c[0] + r[0]).dtype``.
See Also -------- toeplitz : Toeplitz matrix circulant : circulant matrix
Examples -------- >>> from scipy.linalg import hankel >>> hankel([1, 17, 99]) array([[ 1, 17, 99], [17, 99, 0], [99, 0, 0]]) >>> hankel([1,2,3,4], [4,7,7,8,9]) array([[1, 2, 3, 4, 7], [2, 3, 4, 7, 7], [3, 4, 7, 7, 8], [4, 7, 7, 8, 9]])
""" c = np.asarray(c).ravel() if r is None: r = np.zeros_like(c) else: r = np.asarray(r).ravel() # Form a 1D array of values to be used in the matrix, containing `c` # followed by r[1:]. vals = np.concatenate((c, r[1:])) # Stride on concatenated array to get hankel matrix out_shp = len(c), len(r) n = vals.strides[0] return as_strided(vals, shape=out_shp, strides=(n, n)).copy()
""" Construct a Hadamard matrix.
Constructs an n-by-n Hadamard matrix, using Sylvester's construction. `n` must be a power of 2.
Parameters ---------- n : int The order of the matrix. `n` must be a power of 2. dtype : dtype, optional The data type of the array to be constructed.
Returns ------- H : (n, n) ndarray The Hadamard matrix.
Notes ----- .. versionadded:: 0.8.0
Examples -------- >>> from scipy.linalg import hadamard >>> hadamard(2, dtype=complex) array([[ 1.+0.j, 1.+0.j], [ 1.+0.j, -1.-0.j]]) >>> hadamard(4) array([[ 1, 1, 1, 1], [ 1, -1, 1, -1], [ 1, 1, -1, -1], [ 1, -1, -1, 1]])
"""
# This function is a slightly modified version of the # function contributed by Ivo in ticket #675.
if n < 1: lg2 = 0 else: lg2 = int(math.log(n, 2)) if 2 ** lg2 != n: raise ValueError("n must be an positive integer, and n must be " "a power of 2")
H = np.array([[1]], dtype=dtype)
# Sylvester's construction for i in range(0, lg2): H = np.vstack((np.hstack((H, H)), np.hstack((H, -H))))
return H
""" Create a Leslie matrix.
Given the length n array of fecundity coefficients `f` and the length n-1 array of survival coefficients `s`, return the associated Leslie matrix.
Parameters ---------- f : (N,) array_like The "fecundity" coefficients. s : (N-1,) array_like The "survival" coefficients, has to be 1-D. The length of `s` must be one less than the length of `f`, and it must be at least 1.
Returns ------- L : (N, N) ndarray The array is zero except for the first row, which is `f`, and the first sub-diagonal, which is `s`. The data-type of the array will be the data-type of ``f[0]+s[0]``.
Notes ----- .. versionadded:: 0.8.0
The Leslie matrix is used to model discrete-time, age-structured population growth [1]_ [2]_. In a population with `n` age classes, two sets of parameters define a Leslie matrix: the `n` "fecundity coefficients", which give the number of offspring per-capita produced by each age class, and the `n` - 1 "survival coefficients", which give the per-capita survival rate of each age class.
References ---------- .. [1] P. H. Leslie, On the use of matrices in certain population mathematics, Biometrika, Vol. 33, No. 3, 183--212 (Nov. 1945) .. [2] P. H. Leslie, Some further notes on the use of matrices in population mathematics, Biometrika, Vol. 35, No. 3/4, 213--245 (Dec. 1948)
Examples -------- >>> from scipy.linalg import leslie >>> leslie([0.1, 2.0, 1.0, 0.1], [0.2, 0.8, 0.7]) array([[ 0.1, 2. , 1. , 0.1], [ 0.2, 0. , 0. , 0. ], [ 0. , 0.8, 0. , 0. ], [ 0. , 0. , 0.7, 0. ]])
""" f = np.atleast_1d(f) s = np.atleast_1d(s) if f.ndim != 1: raise ValueError("Incorrect shape for f. f must be one-dimensional") if s.ndim != 1: raise ValueError("Incorrect shape for s. s must be one-dimensional") if f.size != s.size + 1: raise ValueError("Incorrect lengths for f and s. The length" " of s must be one less than the length of f.") if s.size == 0: raise ValueError("The length of s must be at least 1.")
tmp = f[0] + s[0] n = f.size a = np.zeros((n, n), dtype=tmp.dtype) a[0] = f a[list(range(1, n)), list(range(0, n - 1))] = s return a
""" Kronecker product.
The result is the block matrix::
a[0,0]*b a[0,1]*b ... a[0,-1]*b a[1,0]*b a[1,1]*b ... a[1,-1]*b ... a[-1,0]*b a[-1,1]*b ... a[-1,-1]*b
Parameters ---------- a : (M, N) ndarray Input array b : (P, Q) ndarray Input array
Returns ------- A : (M*P, N*Q) ndarray Kronecker product of `a` and `b`.
Examples -------- >>> from numpy import array >>> from scipy.linalg import kron >>> kron(array([[1,2],[3,4]]), array([[1,1,1]])) array([[1, 1, 1, 2, 2, 2], [3, 3, 3, 4, 4, 4]])
""" if not a.flags['CONTIGUOUS']: a = np.reshape(a, a.shape) if not b.flags['CONTIGUOUS']: b = np.reshape(b, b.shape) o = np.outer(a, b) o = o.reshape(a.shape + b.shape) return np.concatenate(np.concatenate(o, axis=1), axis=1)
""" Create a block diagonal matrix from provided arrays.
Given the inputs `A`, `B` and `C`, the output will have these arrays arranged on the diagonal::
[[A, 0, 0], [0, B, 0], [0, 0, C]]
Parameters ---------- A, B, C, ... : array_like, up to 2-D Input arrays. A 1-D array or array_like sequence of length `n` is treated as a 2-D array with shape ``(1,n)``.
Returns ------- D : ndarray Array with `A`, `B`, `C`, ... on the diagonal. `D` has the same dtype as `A`.
Notes ----- If all the input arrays are square, the output is known as a block diagonal matrix.
Empty sequences (i.e., array-likes of zero size) will not be ignored. Noteworthy, both [] and [[]] are treated as matrices with shape ``(1,0)``.
Examples -------- >>> from scipy.linalg import block_diag >>> A = [[1, 0], ... [0, 1]] >>> B = [[3, 4, 5], ... [6, 7, 8]] >>> C = [[7]] >>> P = np.zeros((2, 0), dtype='int32') >>> block_diag(A, B, C) array([[1, 0, 0, 0, 0, 0], [0, 1, 0, 0, 0, 0], [0, 0, 3, 4, 5, 0], [0, 0, 6, 7, 8, 0], [0, 0, 0, 0, 0, 7]]) >>> block_diag(A, P, B, C) array([[1, 0, 0, 0, 0, 0], [0, 1, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0], [0, 0, 3, 4, 5, 0], [0, 0, 6, 7, 8, 0], [0, 0, 0, 0, 0, 7]]) >>> block_diag(1.0, [2, 3], [[4, 5], [6, 7]]) array([[ 1., 0., 0., 0., 0.], [ 0., 2., 3., 0., 0.], [ 0., 0., 0., 4., 5.], [ 0., 0., 0., 6., 7.]])
""" if arrs == (): arrs = ([],) arrs = [np.atleast_2d(a) for a in arrs]
bad_args = [k for k in range(len(arrs)) if arrs[k].ndim > 2] if bad_args: raise ValueError("arguments in the following positions have dimension " "greater than 2: %s" % bad_args)
shapes = np.array([a.shape for a in arrs]) out_dtype = np.find_common_type([arr.dtype for arr in arrs], []) out = np.zeros(np.sum(shapes, axis=0), dtype=out_dtype)
r, c = 0, 0 for i, (rr, cc) in enumerate(shapes): out[r:r + rr, c:c + cc] = arrs[i] r += rr c += cc return out
""" Create a companion matrix.
Create the companion matrix [1]_ associated with the polynomial whose coefficients are given in `a`.
Parameters ---------- a : (N,) array_like 1-D array of polynomial coefficients. The length of `a` must be at least two, and ``a[0]`` must not be zero.
Returns ------- c : (N-1, N-1) ndarray The first row of `c` is ``-a[1:]/a[0]``, and the first sub-diagonal is all ones. The data-type of the array is the same as the data-type of ``1.0*a[0]``.
Raises ------ ValueError If any of the following are true: a) ``a.ndim != 1``; b) ``a.size < 2``; c) ``a[0] == 0``.
Notes ----- .. versionadded:: 0.8.0
References ---------- .. [1] R. A. Horn & C. R. Johnson, *Matrix Analysis*. Cambridge, UK: Cambridge University Press, 1999, pp. 146-7.
Examples -------- >>> from scipy.linalg import companion >>> companion([1, -10, 31, -30]) array([[ 10., -31., 30.], [ 1., 0., 0.], [ 0., 1., 0.]])
"""
raise ValueError("Incorrect shape for `a`. `a` must be " "one-dimensional.")
raise ValueError("The length of `a` must be at least 2.")
raise ValueError("The first coefficient in `a` must not be zero.")
""" Create a Helmert matrix of order `n`.
This has applications in statistics, compositional or simplicial analysis, and in Aitchison geometry.
Parameters ---------- n : int The size of the array to create. full : bool, optional If True the (n, n) ndarray will be returned. Otherwise the submatrix that does not include the first row will be returned. Default: False.
Returns ------- M : ndarray The Helmert matrix. The shape is (n, n) or (n-1, n) depending on the `full` argument.
Examples -------- >>> from scipy.linalg import helmert >>> helmert(5, full=True) array([[ 0.4472136 , 0.4472136 , 0.4472136 , 0.4472136 , 0.4472136 ], [ 0.70710678, -0.70710678, 0. , 0. , 0. ], [ 0.40824829, 0.40824829, -0.81649658, 0. , 0. ], [ 0.28867513, 0.28867513, 0.28867513, -0.8660254 , 0. ], [ 0.2236068 , 0.2236068 , 0.2236068 , 0.2236068 , -0.89442719]])
""" H = np.tril(np.ones((n, n)), -1) - np.diag(np.arange(n)) d = np.arange(n) * np.arange(1, n+1) H[0] = 1 d[0] = n H_full = H / np.sqrt(d)[:, np.newaxis] if full: return H_full else: return H_full[1:]
""" Create a Hilbert matrix of order `n`.
Returns the `n` by `n` array with entries `h[i,j] = 1 / (i + j + 1)`.
Parameters ---------- n : int The size of the array to create.
Returns ------- h : (n, n) ndarray The Hilbert matrix.
See Also -------- invhilbert : Compute the inverse of a Hilbert matrix.
Notes ----- .. versionadded:: 0.10.0
Examples -------- >>> from scipy.linalg import hilbert >>> hilbert(3) array([[ 1. , 0.5 , 0.33333333], [ 0.5 , 0.33333333, 0.25 ], [ 0.33333333, 0.25 , 0.2 ]])
""" values = 1.0 / (1.0 + np.arange(2 * n - 1)) h = hankel(values[:n], r=values[n - 1:]) return h
""" Compute the inverse of the Hilbert matrix of order `n`.
The entries in the inverse of a Hilbert matrix are integers. When `n` is greater than 14, some entries in the inverse exceed the upper limit of 64 bit integers. The `exact` argument provides two options for dealing with these large integers.
Parameters ---------- n : int The order of the Hilbert matrix. exact : bool, optional If False, the data type of the array that is returned is np.float64, and the array is an approximation of the inverse. If True, the array is the exact integer inverse array. To represent the exact inverse when n > 14, the returned array is an object array of long integers. For n <= 14, the exact inverse is returned as an array with data type np.int64.
Returns ------- invh : (n, n) ndarray The data type of the array is np.float64 if `exact` is False. If `exact` is True, the data type is either np.int64 (for n <= 14) or object (for n > 14). In the latter case, the objects in the array will be long integers.
See Also -------- hilbert : Create a Hilbert matrix.
Notes ----- .. versionadded:: 0.10.0
Examples -------- >>> from scipy.linalg import invhilbert >>> invhilbert(4) array([[ 16., -120., 240., -140.], [ -120., 1200., -2700., 1680.], [ 240., -2700., 6480., -4200.], [ -140., 1680., -4200., 2800.]]) >>> invhilbert(4, exact=True) array([[ 16, -120, 240, -140], [ -120, 1200, -2700, 1680], [ 240, -2700, 6480, -4200], [ -140, 1680, -4200, 2800]], dtype=int64) >>> invhilbert(16)[7,7] 4.2475099528537506e+19 >>> invhilbert(16, exact=True)[7,7] 42475099528537378560L
""" from scipy.special import comb if exact: if n > 14: dtype = object else: dtype = np.int64 else: dtype = np.float64 invh = np.empty((n, n), dtype=dtype) for i in xrange(n): for j in xrange(0, i + 1): s = i + j invh[i, j] = ((-1) ** s * (s + 1) * comb(n + i, n - j - 1, exact) * comb(n + j, n - i - 1, exact) * comb(s, i, exact) ** 2) if i != j: invh[j, i] = invh[i, j] return invh
""" Returns the n x n Pascal matrix.
The Pascal matrix is a matrix containing the binomial coefficients as its elements.
Parameters ---------- n : int The size of the matrix to create; that is, the result is an n x n matrix. kind : str, optional Must be one of 'symmetric', 'lower', or 'upper'. Default is 'symmetric'. exact : bool, optional If `exact` is True, the result is either an array of type numpy.uint64 (if n < 35) or an object array of Python long integers. If `exact` is False, the coefficients in the matrix are computed using `scipy.special.comb` with `exact=False`. The result will be a floating point array, and the values in the array will not be the exact coefficients, but this version is much faster than `exact=True`.
Returns ------- p : (n, n) ndarray The Pascal matrix.
See Also -------- invpascal
Notes ----- See http://en.wikipedia.org/wiki/Pascal_matrix for more information about Pascal matrices.
.. versionadded:: 0.11.0
Examples -------- >>> from scipy.linalg import pascal >>> pascal(4) array([[ 1, 1, 1, 1], [ 1, 2, 3, 4], [ 1, 3, 6, 10], [ 1, 4, 10, 20]], dtype=uint64) >>> pascal(4, kind='lower') array([[1, 0, 0, 0], [1, 1, 0, 0], [1, 2, 1, 0], [1, 3, 3, 1]], dtype=uint64) >>> pascal(50)[-1, -1] 25477612258980856902730428600L >>> from scipy.special import comb >>> comb(98, 49, exact=True) 25477612258980856902730428600L
"""
from scipy.special import comb if kind not in ['symmetric', 'lower', 'upper']: raise ValueError("kind must be 'symmetric', 'lower', or 'upper'")
if exact: if n >= 35: L_n = np.empty((n, n), dtype=object) L_n.fill(0) else: L_n = np.zeros((n, n), dtype=np.uint64) for i in range(n): for j in range(i + 1): L_n[i, j] = comb(i, j, exact=True) else: L_n = comb(*np.ogrid[:n, :n])
if kind == 'lower': p = L_n elif kind == 'upper': p = L_n.T else: p = np.dot(L_n, L_n.T)
return p
""" Returns the inverse of the n x n Pascal matrix.
The Pascal matrix is a matrix containing the binomial coefficients as its elements.
Parameters ---------- n : int The size of the matrix to create; that is, the result is an n x n matrix. kind : str, optional Must be one of 'symmetric', 'lower', or 'upper'. Default is 'symmetric'. exact : bool, optional If `exact` is True, the result is either an array of type `numpy.int64` (if `n` <= 35) or an object array of Python integers. If `exact` is False, the coefficients in the matrix are computed using `scipy.special.comb` with `exact=False`. The result will be a floating point array, and for large `n`, the values in the array will not be the exact coefficients.
Returns ------- invp : (n, n) ndarray The inverse of the Pascal matrix.
See Also -------- pascal
Notes -----
.. versionadded:: 0.16.0
References ---------- .. [1] "Pascal matrix", http://en.wikipedia.org/wiki/Pascal_matrix .. [2] Cohen, A. M., "The inverse of a Pascal matrix", Mathematical Gazette, 59(408), pp. 111-112, 1975.
Examples -------- >>> from scipy.linalg import invpascal, pascal >>> invp = invpascal(5) >>> invp array([[ 5, -10, 10, -5, 1], [-10, 30, -35, 19, -4], [ 10, -35, 46, -27, 6], [ -5, 19, -27, 17, -4], [ 1, -4, 6, -4, 1]])
>>> p = pascal(5) >>> p.dot(invp) array([[ 1., 0., 0., 0., 0.], [ 0., 1., 0., 0., 0.], [ 0., 0., 1., 0., 0.], [ 0., 0., 0., 1., 0.], [ 0., 0., 0., 0., 1.]])
An example of the use of `kind` and `exact`:
>>> invpascal(5, kind='lower', exact=False) array([[ 1., -0., 0., -0., 0.], [-1., 1., -0., 0., -0.], [ 1., -2., 1., -0., 0.], [-1., 3., -3., 1., -0.], [ 1., -4., 6., -4., 1.]])
""" from scipy.special import comb
if kind not in ['symmetric', 'lower', 'upper']: raise ValueError("'kind' must be 'symmetric', 'lower' or 'upper'.")
if kind == 'symmetric': if exact: if n > 34: dt = object else: dt = np.int64 else: dt = np.float64 invp = np.empty((n, n), dtype=dt) for i in range(n): for j in range(0, i + 1): v = 0 for k in range(n - i): v += comb(i + k, k, exact=exact) * comb(i + k, i + k - j, exact=exact) invp[i, j] = (-1)**(i - j) * v if i != j: invp[j, i] = invp[i, j] else: # For the 'lower' and 'upper' cases, we computer the inverse by # changing the sign of every other diagonal of the pascal matrix. invp = pascal(n, kind=kind, exact=exact) if invp.dtype == np.uint64: # This cast from np.uint64 to int64 OK, because if `kind` is not # "symmetric", the values in invp are all much less than 2**63. invp = invp.view(np.int64)
# The toeplitz matrix has alternating bands of 1 and -1. invp *= toeplitz((-1)**np.arange(n)).astype(invp.dtype)
return invp
""" Discrete Fourier transform matrix.
Create the matrix that computes the discrete Fourier transform of a sequence [1]_. The n-th primitive root of unity used to generate the matrix is exp(-2*pi*i/n), where i = sqrt(-1).
Parameters ---------- n : int Size the matrix to create. scale : str, optional Must be None, 'sqrtn', or 'n'. If `scale` is 'sqrtn', the matrix is divided by `sqrt(n)`. If `scale` is 'n', the matrix is divided by `n`. If `scale` is None (the default), the matrix is not normalized, and the return value is simply the Vandermonde matrix of the roots of unity.
Returns ------- m : (n, n) ndarray The DFT matrix.
Notes ----- When `scale` is None, multiplying a vector by the matrix returned by `dft` is mathematically equivalent to (but much less efficient than) the calculation performed by `scipy.fftpack.fft`.
.. versionadded:: 0.14.0
References ---------- .. [1] "DFT matrix", http://en.wikipedia.org/wiki/DFT_matrix
Examples -------- >>> from scipy.linalg import dft >>> np.set_printoptions(precision=5, suppress=True) >>> x = np.array([1, 2, 3, 0, 3, 2, 1, 0]) >>> m = dft(8) >>> m.dot(x) # Compute the DFT of x array([ 12.+0.j, -2.-2.j, 0.-4.j, -2.+2.j, 4.+0.j, -2.-2.j, -0.+4.j, -2.+2.j])
Verify that ``m.dot(x)`` is the same as ``fft(x)``.
>>> from scipy.fftpack import fft >>> fft(x) # Same result as m.dot(x) array([ 12.+0.j, -2.-2.j, 0.-4.j, -2.+2.j, 4.+0.j, -2.-2.j, 0.+4.j, -2.+2.j]) """ if scale not in [None, 'sqrtn', 'n']: raise ValueError("scale must be None, 'sqrtn', or 'n'; " "%r is not valid." % (scale,))
omegas = np.exp(-2j * np.pi * np.arange(n) / n).reshape(-1, 1) m = omegas ** np.arange(n) if scale == 'sqrtn': m /= math.sqrt(n) elif scale == 'n': m /= n return m |