{"action":"created","issue":{"url":"https://api.github.com/repos/trthatcher/MLKernels.jl/issues/26","labels_url":"https://api.github.com/repos/trthatcher/MLKernels.jl/issues/26/labels{/name}","comments_url":"https://api.github.com/repos/trthatcher/MLKernels.jl/issues/26/comments","events_url":"https://api.github.com/repos/trthatcher/MLKernels.jl/issues/26/events","html_url":"https://github.com/trthatcher/MLKernels.jl/issues/26","id":81836469,"number":26,"title":"Definity of kernels","user":{"login":"st--","id":5763727,"avatar_url":"https://avatars.githubusercontent.com/u/5763727?v=3","gravatar_id":"","url":"https://api.github.com/users/st--","html_url":"https://github.com/st--","followers_url":"https://api.github.com/users/st--/followers","following_url":"https://api.github.com/users/st--/following{/other_user}","gists_url":"https://api.github.com/users/st--/gists{/gist_id}","starred_url":"https://api.github.com/users/st--/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/st--/subscriptions","organizations_url":"https://api.github.com/users/st--/orgs","repos_url":"https://api.github.com/users/st--/repos","events_url":"https://api.github.com/users/st--/events{/privacy}","received_events_url":"https://api.github.com/users/st--/received_events","type":"User","site_admin":false},"labels":[],"state":"open","locked":false,"assignee":null,"milestone":null,"comments":1,"created_at":"2015-05-28T09:00:45Z","updated_at":"2015-05-30T20:52:41Z","closed_at":null,"body":"I had assumed the Sigmoid kernel was c.p.d., but according to the [paper I just read](http://vip.uwaterloo.ca/files/publications/Carrington%20et%20al%20-%20A%20New%20Mercer%20Sigmoid%20Kernel.pdf) about the Mercer Sigmoid kernel apparently that's only true for some parameter values, depending on the data set... so I suppose we should have iscondposdef(::SigmoidKernel) = false?\r\n\r\nAlso, I think we might've been working with `isposdef(::Kernel)` meaning \"positive semi-definite\", but Base.isposdef is \"strictly positive definite\" (e.g. isposdef(0) = false...), and if we extend a Base function we should follow that. For kernels it doesn't seem to make much difference whether it's positive semi-definite or strictly positive definite, so maybe move to our own function? We could have ismercer()... as positive semi-definity is sufficient and necessary for a kernel being a Mercer kernel."},"comment":{"url":"https://api.github.com/repos/trthatcher/MLKernels.jl/issues/comments/107084561","html_url":"https://github.com/trthatcher/MLKernels.jl/issues/26#issuecomment-107084561","issue_url":"https://api.github.com/repos/trthatcher/MLKernels.jl/issues/26","id":107084561,"user":{"login":"trthatcher","id":4790109,"avatar_url":"https://avatars.githubusercontent.com/u/4790109?v=3","gravatar_id":"","url":"https://api.github.com/users/trthatcher","html_url":"https://github.com/trthatcher","followers_url":"https://api.github.com/users/trthatcher/followers","following_url":"https://api.github.com/users/trthatcher/following{/other_user}","gists_url":"https://api.github.com/users/trthatcher/gists{/gist_id}","starred_url":"https://api.github.com/users/trthatcher/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/trthatcher/subscriptions","organizations_url":"https://api.github.com/users/trthatcher/orgs","repos_url":"https://api.github.com/users/trthatcher/repos","events_url":"https://api.github.com/users/trthatcher/events{/privacy}","received_events_url":"https://api.github.com/users/trthatcher/received_events","type":"User","site_admin":false},"created_at":"2015-05-30T20:52:41Z","updated_at":"2015-05-30T20:52:41Z","body":"Interesting. I think an `ismercer` function is good idea - pragmatically speaking, most users would only need to know if the kernel is a Mercer kernel or not. Some algorithms require the kernel to be Mercer for convergence while others do not.\r\n\r\nI made the changes in the last commit: 50d68d80bfc89c3cd22694a0302ca81bcd82207b"}}