2017-12-05 11:34:10 +0200 | received badge | ● Student (source) |

2017-12-05 06:58:15 +0200 | asked a question | svd is here a sage command to reverse the SVD, that ia restore the original matrix? SVD matrix analysis |

2017-10-29 19:52:43 +0200 | commented question | integral command fails: copy, paste and run. Then exchange m(x,y) and n(x,y) and rerun. I get Maxima error I did not know about reposting. I used no other name. I did not know how to write sage code for predator/model and I came to ask.sagemath and now I know. |

2017-10-29 19:47:05 +0200 | commented question | integral command fails: copy, paste and run. Then exchange m(x,y) and n(x,y) and rerun. I get Maxima error dan; I do not know how to write the integral that is not evaluated, that is; how to plug in (what is plugged in) into the mix,m(x,y) and n(x,y) terms in the integral command. Mathematica handles the problem and sage does not in the way I wrote the code. I assume my code is at fault ;but do not know where. I lost my Mathematica when I went to a 64 bit machine and too expensive to buy again on SS pension; hence I am learning sage. I am not a mathematician. I am nearly 80 and math is a hobby. So, perhaps I don't know what I am doing. If I did I maybe I would not need 'ask.sagemath |

2017-10-29 19:11:27 +0200 | commented question | integral fails entering code below. Hopefully I'll avoid typos thank you. I submitted another version with your instructions |

2017-10-29 18:42:10 +0200 | received badge | ● Associate Editor (source) |

2017-10-29 17:54:06 +0200 | asked a question | integral command fails: copy, paste and run. Then exchange m(x,y) and n(x,y) and rerun. I get Maxima error show("G.2.a singularity across sinks and swirls from Mathematica Text") show("second try to enter workable code that can be copied, pasted and run") var('y a b q t r') m(x,y)=q*(x-a)/((x-a)^2+(y-b)^2) n(x,y)=q*(y-b)/((x-a)^2+(y-b)^2) """ replacments get Maxima error in integrate command m(x,y)=5*(x-a)/sqrt((x-a)^2+(y-b)^2) n(x,y)=5*(y-b)/sqrt((x-a)^2+(y-b)^2) I have other examples of m(x,y) and n(x,y) that fail """ show("n(x,y)=",n(x,y)) show("Field(x,y)=(m(x,y),n(x,y))") Field=(m(x,y),n(x,y)) show("Field=",Field) singularity=(a,b) show("singularity=(a,b), when a==x and b==y") show("----------------") show("constructing small circle centered at singularity (a,b) with radius 'r'") show("aiming to determine if the singularity is a source of flow, sink or neither") xr(t)=singularity[0]+r*cos(t) yr(t)=singularity[1]+r*sin(t) show("xr(t)=",xr(t)) show("yr(t)=",yr(t)) xrprime(t)=diff(xr(t),t) yrprime(t)=diff(yr(t),t) show("xrprime(t)=",xrprime(t)," = derivative(xr(t),t)") show("yrprime(t)=",yrprime(t)," = derivative(yr(t),t)") show("----------------") show("measure flow across the circle surrounding the singularity") show("integral=integral(-n(xr(t),yr(t)) * xrprime(t)+m(xr(t),yr(t)) * yrprime(t),(t,0,2*pi))") integral=integral(-n(xr(t),yr(t)) * xrprime(t)+m(xr(t),yr(t)) * yrprime(t),(t,0,2*pi)) show("integral=",integral) show("q>0 then singularity at (a,b) is a source of flow ") show("q<0 then singularity at (a,b) is a sink for flow") show("q=0 then no net flow at singularity (a,b)") show("----------------") show("now try to run the above with the replacement m(x,y), n(x,y) :") show("I get an error in Maxima, see below") show("these m(x,y) and n(x,y) do not work for me") show("m(x,y)=5*(x-a)/sqrt((x-a)^2+(y-b)^2)") show("n(x,y)=5*(y-b)/sqrt((x-a)^2+(y-b)^2)") show("In sage I get RuntimeError: ECL says: Error executing code in Maxima: atanh: argument 1 isn't in the") show("domain of atanh.") show("Mathematica's correct answer is: integral=flow=10pisqrt(r^2)") show("thank you") |

2017-10-29 16:48:11 +0200 | asked a question | retry at the integration: copy and paste show("I hope that now you can copy and paste") show("G.2.a singularity across sinks and swirls from mathematica text") var('y a b q t r') m(x,y)=q*(x-a)/((x-a)^2+(y-b)^2) n(x,y)=q*(y-b)/((x-a)^2+(y-b)^2) """ replace m(x,y and n(x,y) above with the entries just below and rerun to see the error in Maxima m(x,y)=5*(x-a)/sqrt((x-a)^2+(y-b)^2) n(x,y)=5*(y-b)/sqrt((x-a)^2+(y-b)^2) the error occurs in the line" 'integral=integral(-n(xr(t),yr(t)) there are other examples of m(x,y) and n(x,y) that also fail in the integration command """ show("n(x,y)=",n(x,y)) show("n(x,y)=",n(x,y)) show("Field(x,y)=(m(x,y),n(x,y))") Field=(m(x,y),n(x,y)) show("Field=",Field) singularity=(a,b) show("singularity=(a,b), when a==x and b==y") show("----------------") show("constructing small circle centered at singularity (a,b) with radius 'r' ") show("aiming to test the singularity. Is the singularity a source of flow, a sink or neither?") xr(t)=singularity[0]+r*cos(t) yr(t)=singularity[1]+r*sin(t) show("xr(t)=",xr(t)) show("yr(t)=",yr(t)) xrprime(t)=diff(xr(t),t) yrprime(t)=diff(yr(t),t) show("xrprime(t)=",xrprime(t)," = derivative(xr(t),t)") show("yrprime(t)=",yrprime(t)," = derivative(yr(t),t)") show("----------------") show("measure flow across the circle surrounding the singularity") show("integral=integral(-n(xr(t),yr(t)) show("and the integral command fails in Maxima when m(x,y) and n(x,y) are replaced with the other m(x,y), n(x,y)") show("integral=integral(-n(xr(t),yr(t)) show("integral=",integral) show("q>0 then singularity at (a,b) is a source of flow ") show("q<0 then singularity at (a,b) is a sink for flow") show("q=0 then no net flow at singularity (a,b)") show("----------------") show("now try to run the above with the replacement m(x,y), n(x,y) above. The error in Maxima comes up:") show("in the line integral=integral(-n(xr(t),yr(t)) show("does not work for me") m(x,y)=5*(x-a)/sqrt((x-a)^2+(y-b)^2) n(x,y)=5*(y-b)/sqrt((x-a)^2+(y-b)^2) show("In sage I get RuntimeError: ECL says: Error executing code in Maxima: atanh: argument 1 isn't in the domain of ") show("atanh.") show("Mathematica's correct answer is: integral=flow at singularity=10 show("Mathematica's correct answer is: integral=flow at singularity=10 show("hence in the limit r->0 there is no flow at the singularity and the singularity is neither a source nor a sink") show("thank you for your help") show("Meir") |

2017-10-29 14:56:40 +0200 | asked a question | integral fails entering code below. Hopefully I'll avoid typos Here is the code: |

2017-10-29 14:42:25 +0200 | commented question | integration Thank you. I'll try again |

2017-10-29 02:38:01 +0200 | received badge | ● Editor (source) |

2017-10-29 02:29:56 +0200 | asked a question | integration show("G.2.a singularity across sinks and swirls")
show("copy and paste the preview text and ignore the rest")
show("then remove leading '#' in line 5 and 6 and rerun with error")
show("runs in mathematica and is from a mathematica text")
show("other examples where maxima fails")
var('y a b q t r')
m(x,y)=q ## m(x,y)=5*(x-a)/sqrt((x-a)^2+(y-b)^2)## n(x,y)=5*(y-b)/sqrt((x-a)^2+(y-b)^2)show("m(x,y)=",m(x,y))
show("n(x,y)=",n(x,y))
show("Field(x,y)=(m(x,y),n(x,y))")
Field=(m(x,y),n(x,y))
show("Field=",Field)
singularity=(a,b)
show("singularity=(a,b), when a==x and b==y")
show("----------------")
show("constructing small circle centered at singularity (a,b) with radius 'r'")
xr(t)=singularity[0]+r |

2017-09-18 18:48:49 +0200 | commented answer | solution to logistic differential y'=r*y*(1-y)/b dan_fulea: Thank you. Solved my problem/question. I am a senior (very) not a student. Math is my hobby. My mathematica version does not run on a 64 bit machine and therefore, switching to sage and learning on my own with the help of a few texts books. I have not seen/learned the code "sol.subs({y(x):z}).solve(z,to_poly_solve=True)[0].rhs() in any of my text books. After your help I found with 'solve? evaluate' but no explanation as to why. A bit exotic for my level. The code fixes my problem. Thanks again. כול הכבוד P.S. I miss-typed my original questions. Meant to write (1-y/b); not (1-y)/b. That error caused some unecessary confusion. |

2017-09-18 01:46:48 +0200 | commented answer | solution to logistic differential y'=r*y*(1-y)/b re:logistic function
If I understand your reply correctly, y(x)=3 |

2017-09-18 01:46:48 +0200 | commented answer | solution to logistic differential y'=r*y*(1-y)/b let me study this. thank you |

2017-09-15 22:31:31 +0200 | asked a question | solution to logistic differential y'=r*y*(1-y)/b How to solve this logistic differential in sage? mathematica solution is: Other questions/answers found on the logistic topic were not helpful for this problem |

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