Is it a self plagiarism to reuse a section of my previous paper in a new paper? I am writing an article in which I need to reproduce a section of my previous paper. This section includes standard math formulations and very little text. Although I am including this section with some minor changes but overall structure of section would be same. I will include proper citation but still I am worried if this could be a case of self plagiarism ?

The Journal in which I want to submit my work have following clause on copyrights/permission:


  Authors wishing to include figures, tables, or text passages that have already been published elsewhere are required to obtain permission from the copyright owner(s) for both the print and online format and to include evidence that such permission has been granted when submitting their papers. Any material received without such evidence will be assumed to originate from the authors.


The journal in which my article has been published says like this on copyrights/permission :


  Subscribers may reproduce tables of contents or prepare lists of articles including abstracts for internal
  circulation within their institutions. Permission of the Publisher is required for resale or distribution
  outside the institution and for all other derivative works, including compilations and translations. If
  excerpts from other copyrighted works are included, the author(s) must obtain written permission
  from the copyright owners and credit the source(s) in the article. Elsevier has preprinted forms for
  use by authors in these cases.
  
  As an author you (or your employer or institution) have certain rights to reuse your work. More information.. One more thing that this journal says on permission:
  
  As a general rule, written permission must be obtained from the rightsholder in order to re-use any copyrighted material. Typically the rightsholder of published material is the publisher unless it is explicitly indicated otherwise. Copyrighted material can include figures, illustrations, charts, tables, photographs, and text excerpts. Re-use of any borrowed material must be properly acknowledged, even if it is determined that written permission is not necessary.


PS. My research field is Mathematics.
Technically, it is not self-plagiarism if you were to cite the relevant material, even if it's your own. It would be advisable if to rephrase the section. If considerable amount of detail is repeated from thee pervious paper, it would be better to have consent from your publisher before hand. 

I notice this many times among authors. Those who make a minor improvement over their own research tend to have major portions of their old papers rephrased (some even repeated, but again, that's not encouraged). As long the overall concept is novel this won't be a problem.