# [Numpy-discussion] strange dimension-dependent behaviour of einsum

Charles R Harris charlesr.harris at gmail.com
Tue May 17 21:50:20 EDT 2011

```On Tue, May 17, 2011 at 7:32 PM, Charles R Harris <charlesr.harris at gmail.com
> wrote:

>
>
> On Tue, May 17, 2011 at 6:47 PM, Wieland Brendel <wielandbrendel at gmx.net>wrote:
>
>>
>> Hello,
>> I am encountering a very strange behaviour of einsum on my machine. I
>> tracked the problem down to the following test code:
>>
>> from numpy import *
>>
>> T = random.random((3,10,10))
>> W = random.random((3,10,7,275))
>>
>> print all(einsum('ij...,j...->i...',T[0],W[0]) +
>> einsum('ij...,j...->i...',T[1],W[1]) + einsum('ij...,j...->i...',T[2],W[2])
>> - einsum('pij,pjts->its',T,W)<  1e-10)
>> print sum(abs(einsum('ij...,j...->i...',T[0],W[0]) +
>> einsum('ij...,j...->i...',T[1],W[1]) + einsum('ij...,j...->i...',T[2],W[2])
>> - einsum('pij,pjts->its',T,W)))
>>
>> On my machine the equality is not fulfilled. However, this depends,
>> strange enough, on the dimensions of W: if the last dimension e.g. is
>> 500 instead of 275, things work again and the equality is fulfilled.
>>
>>
> The equality being that the expression should be ~0?
>
> I see the problem when the last index is in the range 235 - 390.
>
>
>> Are you encountering similar problems or is this just my
>> machine/installation?
>>
>>
> Out of curiosity, which machine/OS are you using? I'm on 64 bit fedora 14,
> AMD 940.
>
>
Using

T = ones((2,8,8))
W = ones((2,8,8,i))

The problem occurs for i in the closed ranges 129..204 and 257..341. The
starting points look suspicious but the end points not so much. I think
something isn't getting reset to zero.

Chuck
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