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I'm newbie here, and using Mathematica for two weeks, so, anyone can help me with this? I tried using the Findroot command but I don't get it. Thanks in advance guys.

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2 Answers 2

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f[x_] := BesselJ[0, x]

Plot[f[x], {x, 0, 15},
 Epilog -> {Red, AbsolutePointSize[4],
   Tooltip[Point[{BesselJZero[0, #], 0}], BesselJZero[0, #] // N] & /@
    Range[5]}]

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To see the progressive estimates using Newton-Raphson, use FixedPointList

(sol1a = Table[
    FixedPointList[# - f[#]/f'[#] &, x0], {x0, 3., 15, 3}]) // Column

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To see just the final values use FixedPoint

(sol1b = Table[FixedPoint[# - f[#]/f'[#] &, x0], {x0, 3., 15, 3}])

(* {2.40483, 5.52008, 8.65373, 11.7915, 14.9309} *)

Alternatively, using NestList or Nest

(sol2a = Table[
    NestList[# - f[#]/f'[#] &, x0, 10], {x0, 3., 15., 3.}]) // Column

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(sol2b = Table[Nest[# - f[#]/f'[#] &, x0, 10], {x0, 3., 15., 3.}])

(* {2.40483, 5.52008, 8.65373, 11.7915, 14.9309} *)

Using FindRoot

sol3 = x /. FindRoot[BesselJ[0, x] == 0, {x, #}] & /@ Range[3., 15, 3]

(* {2.40483, 5.52008, 8.65373, 11.7915, 14.9309} *)

Using NSolve

sol4 = x /. NSolve[{BesselJ[0, x] == 0, 0 < x < 15}, x]

(* {2.40483, 5.52008, 8.65373, 11.7915, 14.9309} *)

Solve or Reduce give the solution in terms of BesselJZero

x /. Solve[{BesselJ[0, x] == 0, 0 < x < 15}, x]

(* {BesselJZero[0, 1], BesselJZero[0, 2], BesselJZero[0, 3], BesselJZero[0, 4], 
 BesselJZero[0, 5]} *)

x /. {ToRules[Reduce[{BesselJ[0, x] == 0, 0 < x < 15}, x]]}

(* {BesselJZero[0, 1], BesselJZero[0, 2], BesselJZero[0, 3], BesselJZero[0, 4], 
 BesselJZero[0, 5]} *)

Comparing with the BesselJZero values

bjz = BesselJZero[0, #] & /@ Range[5];

sol1a[[All, -1]] == sol1b == sol2a[[All, -1]] == sol2b == sol3 == sol4 == bjz

(* True *)
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  • $\begingroup$ thanks for the answer, but I do not have a formula, that's what I do not understand, how do I apply it? $\endgroup$
    – shatman
    Commented Aug 20, 2018 at 13:10
  • $\begingroup$ You gave Newton-Raphson formula, i.e., x[i+1] == x[i] - f[x[i]]/f'[x[i]]. When written as a pure function the RHS is # - f[#]/f'[#] &. FixedPointList and FixedPoint iterate this pure function until the results converge and stops automatically. NestList and Nest iterates the pure function a specified number of times. For a sufficiently large number of specified iterations the result will converge to the fixed point. In all of these cases a starting value (x[0]) must be specified that is in the region of the desired root. $\endgroup$
    – Bob Hanlon
    Commented Aug 20, 2018 at 14:35
  • $\begingroup$ thanks Bob, Now I understand it all $\endgroup$
    – shatman
    Commented Aug 20, 2018 at 19:41
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  f[x_] := BesselJ[0, x]    
    x[0] = 1.;(*initial point*)(*different initial point converge to different root*)
    Table[x[i + 1] = x[i] - f[x[i]]/f'[x[i]]; x[i], {i, 0, 10}]

{1., 2.73889, 2.36779, 2.40456, 2.40483, 2.40483, 2.40483, 2.40483,2.40483, 2.40483, 2.40483}

  x[0] = 5.;(*initial point*)
    Table[x[i + 1] = x[i] - f[x[i]]/f'[x[i]]; x[i], {i, 0, 10}]

{5., 5.54215, 5.52003, 5.52008, 5.52008, 5.52008, 5.52008, 5.52008, 5.52008, 5.52008, 5.52008}

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  • $\begingroup$ thank you this was very helpful :) $\endgroup$
    – shatman
    Commented Aug 20, 2018 at 13:12

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