Core strengthening based on pilates

Exercises based on pilates are a good way to strengthen the core.  Here is a 15-min workout with six exercises that are relatively easy to figure out.  They are from on a website by an LA-based pilates instructor.  Her suggestion is to do them 3-4 times a week.

6 Easy Exercises to Strengthen Your Core

Here is an example of a full Pilates workout with good explanations for each exercise.  Also includes both easier and harder variations for many exercises. It can be helpful to learn Pilates techniques in a class to begin with.  Although having a dance background made it easier for me because I remembered a few of the exercises from dance classes long ago. 

30 Minutes Mat Pilates Core Workout For a Great Full Body Workout

https://youtu.be/ywJe6ZJhyA0