Recurring Scheduling Apex Job for every 10,15 etc minutes

Hi All,
I was just trying to schedule my job for every 15 minute by executing below code. But the job was running once only and it was not triggering again.
Schedular_GlobalAppLogPurgeBatch globalPurgeBatchSchedular = new Schedular_GlobalAppLogPurgeBatch();
System.schedule('Global App Logs Purge1', '0 0 15 * * ? *', globalPurgeBatchSchedular);

Then  i came to know that Jobs cannot be scheduled in increments smaller than one hour. To schedule the job fro every 15 minute, i have to do the following
Schedular_GlobalAppLogPurgeBatch globalPurgeBatchSchedular = new Schedular_GlobalAppLogPurgeBatch();
System.schedule('Global App Logs Purge1', '0 0 * * * ? *', globalPurgeBatchSchedular);
System.schedule('Global App Logs Purge2', '0 15 * * * ? *', globalPurgeBatchSchedular);
System.schedule('Global App Logs Purge3', '0 30 * * * ? *', globalPurgeBatchSchedular);
System.schedule('Global App Logs Purge4', '0 45 * * * ? *', globalPurgeBatchSchedular);

So conclusion is that if you want to schedule for less than hour, you should use below formula.
No of System.schedule() call = 60/frequency in minutes;

e.g. For 10 minutes, please run:
No of System.schedule() call :  60/10 = 6

Schedular_GlobalAppLogPurgeBatch globalPurgeBatchSchedular = new Schedular_GlobalAppLogPurgeBatch();
System.schedule('Global App Logs Purge1', '0 0 * * * ? *', globalPurgeBatchSchedular);
System.schedule('Global App Logs Purge2', '0 10 * * * ? *', globalPurgeBatchSchedular);
System.schedule('Global App Logs Purge3', '0 20 * * * ? *', globalPurgeBatchSchedular);
System.schedule('Global App Logs Purge4', '0 30 * * * ? *', globalPurgeBatchSchedular);
System.schedule('Global App Logs Purge5', '0 40 * * * ? *', globalPurgeBatchSchedular);
System.schedule('Global App Logs Purge6', '0 50 * * * ? *', globalPurgeBatchSchedular);

I hope this will help you.