零阶一类贝塞尔函数曲线及其包络线 matlab源程序

废话不多说，请看源程序：

clear ,clc;
format long
x=(0:0.01:100)";
y_0=besselj(0,x);
% y_1=besselj(1,x); %一阶，这里只画了0阶
% y_2=besselj(2,x); %二阶


plot(x,y_0);grid on;
axis([0,100,-1,1]);
title("0阶贝塞尔函数曲线图");
xlabel("Variable X");
ylabel("Variable Y");

%画包络线
hold on;
[up,down] = envelope(x,y_0,"spline");
plot(x, up, "r");
plot(x, down, "r");

其中的envelope是个单独的.m文件，是个函数，用来画包络线。说白了就是求拐点然后插值：

function [up,down] = envelope(x,y,interpMethod)

%ENVELOPE gets the data of upper and down envelope of the known input (x,y).
%
% Input parameters:
% x the abscissa of the given data
% y the ordinate of the given data
% interpMethod the interpolation method
%
% Output parameters:
% up the upper envelope, which has the same length as x.
% down the down envelope, which has the same length as x.
%
% See also DIFF INTERP1

% Designed by: Lei Wang, <WangLeiBox@hotmail.com>, 11-Mar-2003.
% Last Revision: 21-Mar-2003.
% Dept. Mechanical & Aerospace Engineering, NC State University.
% $Revision: 1.1 $ $Date: 3/21/2003 10:33 AM $

if length(x) ~= length(y)
error("Two input data should have the same length.");
end

if (nargin < 2)|(nargin > 3),
error("Please see help for INPUT DATA.");
elseif (nargin == 2)
interpMethod = "linear";
end



% Find the extreme maxim values 
% and the corresponding indexes
%----------------------------------------------------
extrMaxValue = y(find(diff(sign(diff(y)))==-2)+1);
extrMaxIndex = find(diff(sign(diff(y)))==-2)+1;



% Find the extreme minim values 
% and the corresponding indexes
%----------------------------------------------------
extrMinValue = y(find(diff(sign(diff(y)))==+2)+1);
extrMinIndex = find(diff(sign(diff(y)))==+2)+1;



up = extrMaxValue;
up_x = x(extrMaxIndex);

down = extrMinValue;
down_x = x(extrMinIndex);



% Interpolation of the upper/down envelope data
%----------------------------------------------------
up = interp1(up_x,up,x,interpMethod); 
down = interp1(down_x,down,x,interpMethod); 

效果：

源程序下载：http://www.pudn.com/downloads495/sourcecode/windows/other/detail2062657.html

csdn上传资源标签里竟然没有Matlab，桑不起啊。。。。