any longer

[navi]

Which is correct:

1-That equation was something I let slip my memory and did not know any longer.
2-That equation was something I let slip my memory and did not know.

Gratefully,
Navi.

[Phil White]

The first sentence makes more sense, but the second part is redundant.

3. That equation had slipped my memory.

That is the more normal formulation of the idiom "slip my memory" and the sentence is not redundant. Of course, native speakers use redundant formulations all the time, but neither of your sentences 1 and 2 sound like something a native speaker would say.

[Bobinwales]

That equation had slipped my memory.
.
I had forgotten that I knew that equation.

[Wizard of Oz]

.. Navi neither 1. nor 2. is anything that this speaker would say .. the formulation let slip my memory to my ears is awkward and incorrect .. I am aware that in Amlish they use constructions such as "he wrote her last week" which also sounds awkward and incorrect to this Auslish speaker .. I would have to write >>

I had let slip from my memory

WoZ whose memory has lots of slips

[JerrySmile]

These are indeed correct ones:

That equation had slipped my memory.
That equation had slipped from my memory.

For the 2nd cf.
"Nothing that happened to me during that delightful time," Rousseau writes of his stay with "Maman" at Les Charmettes, nothing that I did, said, or thought all the while it lasted, has slipped from my memory."