How to make code for sum of 2 integrals in Mathematica
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27-10-2019 - |
Question
I am trying to quickly solve the following problem:
f[r_] := Sum[(((-1)^n (2 r - 2 n - 7)!!)/(2^n n! (r - 2 n - 1)!))
* x^(r - 2*n - 1),
{n, 0, r/2}];
Nw := Transpose[Table[f[j], {i, 1}, {j, 5, 200, 1}]];
X1 = Integrate[Nw . Transpose[Nw], {x, -1, 1}]
I can get the answer quickly with this code:
$starttime = AbsoluteTime[]; Quiet[LaunchKernels[]];
DIM = 50;
Print["$Version = ", $Version, " ||| ",
"Number of Kernels : ", Length[Kernels[]]];
Nw = Transpose[Table[f[j], {i, 1}, {j, 5, DIM, 1}]];
nw2 = Nw.Transpose[Nw];
Round[First[AbsoluteTiming[nw3 = ParallelMap[Expand, nw2]; ]]]
intrule = (pol_Plus)?(PolynomialQ[#1, x]&) :>
(Select[pol, !FreeQ[#1, x] & ] /.
x^(n_.) /; n > -1 :> ((-1)^n + 1)/(n + 1)) + 2*(pol /. x -> 0)]);
Round[First[AbsoluteTiming[X1 = ParallelTable[row /. intrule, {row, nw3}]; ]]]
X1
Print["overall time needed in seconds: ", Round[AbsoluteTime[] - $starttime]];
But how can I manage this code if I need to solve the following problem, where a and b are known constants?
X1 = a Integrate[Nw.Transpose[Nw], {x, -1, 0.235}]
+ b Integrate[Nw.Transpose[Nw], {x, 0.235,1}];
Solution
Here's a simple function to do definite integrals of polynomials
polyIntegrate[expr_List, {x_, x0_, x1_}] := polyIntegrate[#, {x, x0, x1}]&/@expr
polyIntegrate[expr_, {x_, x0_, x1_}] := Check[Total[#
Table[(x1^(1 + n) - x0^(1 + n))/(1 + n), {n, 0, Length[#] - 1}]
]&[CoefficientList[expr, x]], $Failed, {General::poly}]
On its range of applicability, this is about 100 times faster than using Integrate
. This should be fast enough for your problem. If not, then it could be parallelized.
f[r_] := Sum[(((-1)^n*(2*r - 2*n - 7)!!)/(2^n*n!*(r - 2*n - 1)!))*
x^(r - 2*n - 1), {n, 0, r/2}];
Nw = Transpose[Table[f[j], {i, 1}, {j, 5, 50, 1}]];
a*polyIntegrate[Nw.Transpose[Nw], {x, -1, 0.235}] +
b*polyIntegrate[Nw.Transpose[Nw], {x, 0.235, 1}] // Timing // Short
(* Returns: {7.9405,{{0.0097638 a+0.00293462 b,<<44>>,
-0.0000123978 a+0.0000123978 b},<<44>>,{<<1>>}}} *)
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