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/* mpfr_jn_asympt, mpfr_yn_asympt -- shared code for mpfr_jn and mpfr_yn
Copyright 2007, 2008, 2009, 2010 Free Software Foundation, Inc.
Contributed by the Arenaire and Cacao projects, INRIA.
This file is part of the GNU MPFR Library.
The GNU MPFR Library is free software; you can redistribute it and/or modify
it under the terms of the GNU Lesser General Public License as published by
the Free Software Foundation; either version 3 of the License, or (at your
option) any later version.
The GNU MPFR Library is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
License for more details.
You should have received a copy of the GNU Lesser General Public License
along with the GNU MPFR Library; see the file COPYING.LESSER. If not, see
http://www.gnu.org/licenses/ or write to the Free Software Foundation, Inc.,
51 Franklin St, Fifth Floor, Boston, MA 02110-1301, USA. */
#ifdef MPFR_JN
# define FUNCTION mpfr_jn_asympt
#else
# ifdef MPFR_YN
# define FUNCTION mpfr_yn_asympt
# else
# error "neither MPFR_JN nor MPFR_YN is defined"
# endif
#endif
/* Implements asymptotic expansion for jn or yn (formulae 9.2.5 and 9.2.6
from Abramowitz & Stegun).
Assumes |z| > p log(2)/2, where p is the target precision
(z can be negative only for jn).
Return 0 if the expansion does not converge enough (the value 0 as inexact
flag should not happen for normal input).
*/
static int
FUNCTION (mpfr_ptr res, long n, mpfr_srcptr z, mpfr_rnd_t r)
{
mpfr_t s, c, P, Q, t, iz, err_t, err_s, err_u;
mpfr_prec_t w;
long k;
int inex, stop, diverge = 0;
mpfr_exp_t err2, err;
MPFR_ZIV_DECL (loop);
mpfr_init (c);
w = MPFR_PREC(res) + MPFR_INT_CEIL_LOG2(MPFR_PREC(res)) + 4;
MPFR_ZIV_INIT (loop, w);
for (;;)
{
mpfr_set_prec (c, w);
mpfr_init2 (s, w);
mpfr_init2 (P, w);
mpfr_init2 (Q, w);
mpfr_init2 (t, w);
mpfr_init2 (iz, w);
mpfr_init2 (err_t, 31);
mpfr_init2 (err_s, 31);
mpfr_init2 (err_u, 31);
/* Approximate sin(z) and cos(z). In the following, err <= k means that
the approximate value y and the true value x are related by
y = x * (1 + u)^k with |u| <= 2^(-w), following Higham's method. */
mpfr_sin_cos (s, c, z, MPFR_RNDN);
if (MPFR_IS_NEG(z))
mpfr_neg (s, s, MPFR_RNDN); /* compute jn/yn(|z|), fix sign later */
/* The absolute error on s/c is bounded by 1/2 ulp(1/2) <= 2^(-w-1). */
mpfr_add (t, s, c, MPFR_RNDN);
mpfr_sub (c, s, c, MPFR_RNDN);
mpfr_swap (s, t);
/* now s approximates sin(z)+cos(z), and c approximates sin(z)-cos(z),
with total absolute error bounded by 2^(1-w). */
/* precompute 1/(8|z|) */
mpfr_si_div (iz, MPFR_IS_POS(z) ? 1 : -1, z, MPFR_RNDN); /* err <= 1 */
mpfr_div_2ui (iz, iz, 3, MPFR_RNDN);
/* compute P and Q */
mpfr_set_ui (P, 1, MPFR_RNDN);
mpfr_set_ui (Q, 0, MPFR_RNDN);
mpfr_set_ui (t, 1, MPFR_RNDN); /* current term */
mpfr_set_ui (err_t, 0, MPFR_RNDN); /* error on t */
mpfr_set_ui (err_s, 0, MPFR_RNDN); /* error on P and Q (sum of errors) */
for (k = 1, stop = 0; stop < 4; k++)
{
/* compute next term: t(k)/t(k-1) = (2n+2k-1)(2n-2k+1)/(8kz) */
mpfr_mul_si (t, t, 2 * (n + k) - 1, MPFR_RNDN); /* err <= err_k + 1 */
mpfr_mul_si (t, t, 2 * (n - k) + 1, MPFR_RNDN); /* err <= err_k + 2 */
mpfr_div_ui (t, t, k, MPFR_RNDN); /* err <= err_k + 3 */
mpfr_mul (t, t, iz, MPFR_RNDN); /* err <= err_k + 5 */
/* the relative error on t is bounded by (1+u)^(5k)-1, which is
bounded by 6ku for 6ku <= 0.02: first |5 log(1+u)| <= |5.5u|
for |u| <= 0.15, then |exp(5.5u)-1| <= 6u for |u| <= 0.02. */
mpfr_mul_ui (err_t, t, 6 * k, MPFR_IS_POS(t) ? MPFR_RNDU : MPFR_RNDD);
mpfr_abs (err_t, err_t, MPFR_RNDN); /* exact */
/* the absolute error on t is bounded by err_t * 2^(-w) */
mpfr_abs (err_u, t, MPFR_RNDU);
mpfr_mul_2ui (err_u, err_u, w, MPFR_RNDU); /* t * 2^w */
mpfr_add (err_u, err_u, err_t, MPFR_RNDU); /* max|t| * 2^w */
if (stop >= 2)
{
/* take into account the neglected terms: t * 2^w */
mpfr_div_2ui (err_s, err_s, w, MPFR_RNDU);
if (MPFR_IS_POS(t))
mpfr_add (err_s, err_s, t, MPFR_RNDU);
else
mpfr_sub (err_s, err_s, t, MPFR_RNDU);
mpfr_mul_2ui (err_s, err_s, w, MPFR_RNDU);
stop ++;
}
/* if k is odd, add to Q, otherwise to P */
else if (k & 1)
{
/* if k = 1 mod 4, add, otherwise subtract */
if ((k & 2) == 0)
mpfr_add (Q, Q, t, MPFR_RNDN);
else
mpfr_sub (Q, Q, t, MPFR_RNDN);
/* check if the next term is smaller than ulp(Q): if EXP(err_u)
<= EXP(Q), since the current term is bounded by
err_u * 2^(-w), it is bounded by ulp(Q) */
if (MPFR_EXP(err_u) <= MPFR_EXP(Q))
stop ++;
else
stop = 0;
}
else
{
/* if k = 0 mod 4, add, otherwise subtract */
if ((k & 2) == 0)
mpfr_add (P, P, t, MPFR_RNDN);
else
mpfr_sub (P, P, t, MPFR_RNDN);
/* check if the next term is smaller than ulp(P) */
if (MPFR_EXP(err_u) <= MPFR_EXP(P))
stop ++;
else
stop = 0;
}
mpfr_add (err_s, err_s, err_t, MPFR_RNDU);
/* the sum of the rounding errors on P and Q is bounded by
err_s * 2^(-w) */
/* stop when start to diverge */
if (stop < 2 &&
((MPFR_IS_POS(z) && mpfr_cmp_ui (z, (k + 1) / 2) < 0) ||
(MPFR_IS_NEG(z) && mpfr_cmp_si (z, - ((k + 1) / 2)) > 0)))
{
/* if we have to stop the series because it diverges, then
increasing the precision will most probably fail, since
we will stop to the same point, and thus compute a very
similar approximation */
diverge = 1;
stop = 2; /* force stop */
}
}
/* the sum of the total errors on P and Q is bounded by err_s * 2^(-w) */
/* Now combine: the sum of the rounding errors on P and Q is bounded by
err_s * 2^(-w), and the absolute error on s/c is bounded by 2^(1-w) */
if ((n & 1) == 0) /* n even: P * (sin + cos) + Q (cos - sin) for jn
Q * (sin + cos) + P (sin - cos) for yn */
{
#ifdef MPFR_JN
mpfr_mul (c, c, Q, MPFR_RNDN); /* Q * (sin - cos) */
mpfr_mul (s, s, P, MPFR_RNDN); /* P * (sin + cos) */
#else
mpfr_mul (c, c, P, MPFR_RNDN); /* P * (sin - cos) */
mpfr_mul (s, s, Q, MPFR_RNDN); /* Q * (sin + cos) */
#endif
err = MPFR_EXP(c);
if (MPFR_EXP(s) > err)
err = MPFR_EXP(s);
#ifdef MPFR_JN
mpfr_sub (s, s, c, MPFR_RNDN);
#else
mpfr_add (s, s, c, MPFR_RNDN);
#endif
}
else /* n odd: P * (sin - cos) + Q (cos + sin) for jn,
Q * (sin - cos) - P (cos + sin) for yn */
{
#ifdef MPFR_JN
mpfr_mul (c, c, P, MPFR_RNDN); /* P * (sin - cos) */
mpfr_mul (s, s, Q, MPFR_RNDN); /* Q * (sin + cos) */
#else
mpfr_mul (c, c, Q, MPFR_RNDN); /* Q * (sin - cos) */
mpfr_mul (s, s, P, MPFR_RNDN); /* P * (sin + cos) */
#endif
err = MPFR_EXP(c);
if (MPFR_EXP(s) > err)
err = MPFR_EXP(s);
#ifdef MPFR_JN
mpfr_add (s, s, c, MPFR_RNDN);
#else
mpfr_sub (s, c, s, MPFR_RNDN);
#endif
}
if ((n & 2) != 0)
mpfr_neg (s, s, MPFR_RNDN);
if (MPFR_EXP(s) > err)
err = MPFR_EXP(s);
/* the absolute error on s is bounded by P*err(s/c) + Q*err(s/c)
+ err(P)*(s/c) + err(Q)*(s/c) + 3 * 2^(err - w - 1)
<= (|P|+|Q|) * 2^(1-w) + err_s * 2^(1-w) + 2^err * 2^(1-w),
since |c|, |old_s| <= 2. */
err2 = (MPFR_EXP(P) >= MPFR_EXP(Q)) ? MPFR_EXP(P) + 2 : MPFR_EXP(Q) + 2;
/* (|P| + |Q|) * 2^(1 - w) <= 2^(err2 - w) */
err = MPFR_EXP(err_s) >= err ? MPFR_EXP(err_s) + 2 : err + 2;
/* err_s * 2^(1-w) + 2^old_err * 2^(1-w) <= 2^err * 2^(-w) */
err2 = (err >= err2) ? err + 1 : err2 + 1;
/* now the absolute error on s is bounded by 2^(err2 - w) */
/* multiply by sqrt(1/(Pi*z)) */
mpfr_const_pi (c, MPFR_RNDN); /* Pi, err <= 1 */
mpfr_mul (c, c, z, MPFR_RNDN); /* err <= 2 */
mpfr_si_div (c, MPFR_IS_POS(z) ? 1 : -1, c, MPFR_RNDN); /* err <= 3 */
mpfr_sqrt (c, c, MPFR_RNDN); /* err<=5/2, thus the absolute error is
bounded by 3*u*|c| for |u| <= 0.25 */
mpfr_mul (err_t, c, s, MPFR_SIGN(c)==MPFR_SIGN(s) ? MPFR_RNDU : MPFR_RNDD);
mpfr_abs (err_t, err_t, MPFR_RNDU);
mpfr_mul_ui (err_t, err_t, 3, MPFR_RNDU);
/* 3*2^(-w)*|old_c|*|s| [see below] is bounded by err_t * 2^(-w) */
err2 += MPFR_EXP(c);
/* |old_c| * 2^(err2 - w) [see below] is bounded by 2^(err2-w) */
mpfr_mul (c, c, s, MPFR_RNDN); /* the absolute error on c is bounded by
1/2 ulp(c) + 3*2^(-w)*|old_c|*|s|
+ |old_c| * 2^(err2 - w) */
/* compute err_t * 2^(-w) + 1/2 ulp(c) = (err_t + 2^EXP(c)) * 2^(-w) */
err = (MPFR_EXP(err_t) > MPFR_EXP(c)) ? MPFR_EXP(err_t) + 1 : MPFR_EXP(c) + 1;
/* err_t * 2^(-w) + 1/2 ulp(c) <= 2^(err - w) */
/* now err_t * 2^(-w) bounds 1/2 ulp(c) + 3*2^(-w)*|old_c|*|s| */
err = (err >= err2) ? err + 1 : err2 + 1;
/* the absolute error on c is bounded by 2^(err - w) */
mpfr_clear (s);
mpfr_clear (P);
mpfr_clear (Q);
mpfr_clear (t);
mpfr_clear (iz);
mpfr_clear (err_t);
mpfr_clear (err_s);
mpfr_clear (err_u);
err -= MPFR_EXP(c);
if (MPFR_LIKELY (MPFR_CAN_ROUND (c, w - err, MPFR_PREC(res), r)))
break;
if (diverge != 0)
{
mpfr_set (c, z, r); /* will force inex=0 below, which means the
asymptotic expansion failed */
break;
}
MPFR_ZIV_NEXT (loop, w);
}
MPFR_ZIV_FREE (loop);
inex = (MPFR_IS_POS(z) || ((n & 1) == 0)) ? mpfr_set (res, c, r)
: mpfr_neg (res, c, r);
mpfr_clear (c);
return inex;
}
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