Approx.fpt.density  R Documentation 
Approx.fpt.density
computes values of the approximate firstpassagetime (f.p.t.) density, for conditioned and unconditioned problems.
For the unconditioned case, values of the approximate densities for f.p.t. problems conditioned to suitable values of the initial distribution are also calculated.
Approx.fpt.density(dp, t0, T, id, S, env = NULL, variableStep = TRUE,
from.t0 = FALSE, to.T = FALSE, r = 4000, zeroSlope = 0.01,
p0.tol = 8, k = 3, m = 100, n = 250, p = 0.2, alpha = 1,
skip = TRUE, tol = 0.001, it.max)
dp 
an object of class “diffproc” defining a family of diffusion processes. 
t0, T 
lower and upper limits of the considered time interval. Must be finite. 
id 
a numerical value specifying a fixed initial value of the process in the time instant specified in the 
S 
numerical value of a constant boundary or character string with the mathematical expression of a time dependent boundary. 
env 
a named list of objects of numeric or character type specifying the values of names which occur in
the mathematical expressions in objects 
variableStep 
a logical value indicating whether a variable integration step is used. 
from.t0 
a logical value indicating whether the approximation should be calculated from the lower end of the
interval considered, 
to.T 
a logical value indicating whether the approximation should be calculated to the upper end of the
interval considered, 
r 
number of points at which the FPTL function is evaluated. 
zeroSlope 
maximum slope required to consider that a growing function is constant. 
p0.tol 
controls where the FirstPassageTime Location function begins to increase significantly. 
k 
controls whether the FirstPassageTime Location function decreases very slowly. 
m 
Number of equally spaced values to select from the specified initial distribution in the case of an unconditioned f.p.t. problem. 
n 
Number of points used to determine the integration step in subintervals 
p 
Ratio of n used to determine the integration step in subintervals

alpha 
Parameter used to determine the integration step in subintervals

skip 
a logical value indicating whether the intervals at which the FPTL function is near zero could be avoided. 
tol 
If the cumulative integral of the approximation is greater than or equal to 1  tol the algorithm is stopped. 
it.max 
If the number of iterations required for the approximation process is greater than it.max, the function asks for permission to continue. Defaults to 50000 for a conditioned f.p.t. problem, and 1000000 for an unconditioned f.p.t. problem. 
For an unconditioned f.p.t. problem, this function allows to compute directly the approximate f.p.t. density from an object of class “diffproc”.
In contrast to the approximation of a conditioned f.p.t. density by using the Approx.cfpt.density
function, in this case it is not necessary for users to make
previous calls to the functions FPTL
and summary.FPTL
.
For a diffusion process \{X(t), t_0 \leq t \leq T \}
with nondegenerate initial distribution
the unconditioned f.p.t. variable, through a continuous boundary S(t)
, is defined as
T_{S(t), X(t_0)} = \left\{
\begin{array}{lll}
Inf \ \{ t \geq t_0 \ : \ X(t) > S(t) \} & & if \ X(t_0) < S(t_0) \\[7pt]
Inf \ \{ t \geq t_0 \ : \ X(t) < S(t) \} & & if \ X(t_0) > S(t_0)
\end{array}
\right. ,
Its density function can be obtained by means of the numerical integration, in the range of variation of X(t_0)
,
of the corresponding f.p.t. densities conditioned to values of X(t_0)
, weighted by the initial density function.
For each conditioned problem related to an unconditioned f.p.t. problem, the Approx.fpt.density
function makes internal calls to
FPTL
and summary.fptl
functions, in order to localize each conditioned f.p.t. variable, according to the
zeroSlope
, p0.tol
and k
arguments. Then, the function makes a internall
call to Integration.Steps
function, in order to determine the suitable subintervals and
integration steps to be used to approximate the unconditioned f.p.t. density and conditioned f.p.t. densities according to the variableStep
,
from.t0
, to.T
, n
, p
and alpha
arguments.
From this information, a suitable sequence of time instants in [t_0, T]
for the approximation of the f.p.t. densities is available.
For each time instant t in such sequence, the Approx.fpt.density
function computes the value of the f.p.t. density conditioned to
m
values of the initial distribution (equally spaced in its range of variation), and then it computes the value of the unconditioned f.p.t density.
For the approximation of each conditioned f.p.t density, the numerical procedure proposed by Buonocore et al. (1987),
based on the composite trapezoid method, has been implemented. This method has also been used in the numerical integration for obtaining
the unconditioned f.p.t. density.
The mathematical expression of the boundary S
should be a function of t
and may include the argument t0
,
the name x0
, to refer to an initial value of the process, and the parameters specified in the env
argument.
The function checks if the mathematical expression shows syntax errors and if R can compute its symbolic derivative with respect to t
.
The env
argument is a list of tagged values in name = value
form with name
other than x
, t
, y
and s
.
To name the expression of a function h(u)
as a character string we can use `h(u)` = value
if we want to show its dependence on u
, or h = value
otherwise.
The env
argument is copied into a temporary environment for evaluating the mathematical expressions in objects dp
and S
.
R looks for the objects not found into this temporary environment in the parent.frame() environment.
By default the function does not compute the approximate f.p.t. density from the time instant t_0
, but from a more suitable time instant
provided by the FirstPassageTime Location (FPTL) function. It also uses a variable integration step.
If skip = TRUE
, for each subinterval in which it could be possible to avoid calculating all the conditioned f.p.t. densities,
the function checks the value of the approximate unconditioned f.p.t. density
at the lower end of such subinterval. If it is almost 0, the approximate unconditioned f.p.t. density
calculation is avoided in the subinterval, and it continues from the upper end of the subinterval
considering a zero value of the approximate unconditioned f.p.t. density.
Similarly, if to.T = FALSE
, the function checks the cumulative value of the integral for each upper end of the
subintervals for which it is not possible to avoid the application of the numerical algorithm for each conditioned problem.
If it is greater than or equal to 1  tol
, the approximation procedure
is stopped. In any case, the procedure is stopped at the upper end of the last subinterval,
and if the cumulative value of the integral is less than 1  tol
the function issues a warning.
The Approx.fpt.density
function computes and returns an object of class “fpt.density”. This object
is a threecomponent list:
x 
a sequence of suitable time instants in 
y 
the approximate f.p.t. density function values on the x sequence for the unconditioned or conditioned problem at hand. 
y.x0 
NULL for a conditioned f.p.t. problem or a matrix with the values, by columns, of the approximate f.p.t.
densities conditioned to each considered value 
It also includes six additional attributes:
Call  the unevaluated function call, substituting each object of length 1 (referred 
by name in call) by its value.  
Steps  matrix of subintervals and integration steps to consider for computing the 
approximate f.p.t. density.  
cumIntegral  vector of the values of the cumulative integral of the approximation for 
each subinterval considered.  
skips  a list that contains, for each subinterval, the indexes of the initial values 
for which the calculation of the approximate conditioned f.p.t. densities  
has been avoided.  
CPUTime  matrix of user and system times, by columns, required to approximate 
the density for each subinterval considered, by rows.  
summary.fptl  the object of class “summary.fptl” that results in the internal calls to the 
summary.fptl function which is used as sfptl argument in the internal 

call to the Integration.Steps function. 

x
is the vector result of the concatenation of the sequences of equally spaced values in the suitable subintervals
determined by the Integration.Steps
function.
Patricia RománRomán, Juan J. SerranoPérez and Francisco TorresRuiz.
Buonocore, A., Nobile, A.G. and Ricciardi, L.M. (1987) A new integral equation for the evaluation of firstpassagetime probability densities. Adv. Appl. Probab., 19, 784–800.
Román, P., Serrano, J. J., Torres, F. (2008) Firstpassagetime location function: Application to determine firstpassagetime densities in diffusion processes. Comput. Stat. Data Anal., 52, 4132–4146.
P. RománRomán, J.J. SerranoPérez, F. TorresRuiz. (2012) An R package for an efficient approximation of firstpassagetime densities for diffusion processes based on the FPTL function. Applied Mathematics and Computation, 218, 8408–8428.
P. RománRomán, J.J. SerranoPérez, F. TorresRuiz. (2014) More general problems on firstpassage times for diffusion processes: A new version of the fptdApprox R package. Applied Mathematics and Computation, 244, 432–446.
diffproc
about creation of class “diffproc” objects.
is.fpt.density
to test for objects of class “fpt.density”.
print.fpt.density
to show objects of class “fpt.density”.
report.fpt.density
to generate a report.
plot.fpt.density
for graphical display.
FPTL
to evaluate the FPTL function and create objects of class “fptl”.
summary.fptl
to locate the f.p.t. variable and create objects of class “summary.fptl”.
## Continuing the diffproc(.) example:
## Making an efficient approximation of the f.p.t. density
## in the case of a conditioned f.p.t. problem. (optimal
## variable integration steps and small computational cost)
yyy.cp < Approx.fpt.density(dp = Lognormal, t0 = 0, T = 18, id = 1, S =
"4.5 + 4*t^2 + 7*t*sqrt(t)*sin(6*sqrt(t))",
env = list(m = 0.48, sigma = 0.07))
yyy.cp
## Not run:
## Making a less efficient approximation of the f.p.t. density
## (optimal fixed integration step but high computational cost related to
## the efficient approximation)
yyy1.cp < Approx.fpt.density(dp = Lognormal, t0 = 0, T = 18, id = 1, S =
"4.5 + 4*t^2 + 7*t*sqrt(t)*sin(6*sqrt(t))",
env = list(m = 0.48, sigma = 0.07),
variableStep = FALSE, from.t0 = TRUE, to.T =
TRUE, skip = FALSE)
yyy1.cp
## Making an efficient approximation of the f.p.t. density
## in the case of an unconditioned f.p.t. problem.
yyy.ucp < Approx.fpt.density(dp = Lognormal, t0 = 0, T = 18, id =
list("dlnorm(x,0.005,0.1)", "Lambda(0.005,0.1)",
"\\Lambda(0.005,0.1)", "Lognormal(0.005,0.1)"),
S = "4.5 + 4*t^2 + 7*t*sqrt(t)*sin(6*sqrt(t))",
env = list(m = 0.48, sigma = 0.07), m=25)
yyy.ucp
## End(Not run)
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