Puput Dani'e

 https://mathworld.wolfram.com/Del.html https://sayitwithscience.tumblr.com/post/7790074043/the-del-operator-denoted-with-what-is-called-the

 https://archive.physionet.org/physiobank/database/mitdb/

 https://www.iotstarters.com/smart-doorbell-using-esp32-camera/