In this sequel to Mathematica, the humans, now knowing that everything is mathematics and having been made immortal by the ultimate mathematician, encounter a race of beings somewhere between material and abstract and learn that time does not exist.
Unfortunately, the author's knowledge of science and math (and his writing ability) do not quite live up to the grandiose themes that these stories attempt to address. All of the mathematics discussed is essentially arithmetic and using the word "electron" seems to be all he can do to sound scientific. For instance, the machine which travels between universes is said to do so by "subtraction", and beings are immortal if they are built up of "uncancellable figures". Lester del Rey seemed to agree with this assessment back in 1936 when he wrote:
"[Mathematica Plus] was pseudomathematics, fiction, and hokum. Its appeal lay in a trick of using grandsounding phrases and vague hints which orators have used to thrill the masses since Greece, but this time they were given a semimathematical form. Analyze most of it, and it means nothing...he does a good job of it too, and I enjoy his tricks thoroughly." 
I must beg to differ with that last remark, however. I remained unimpressed and unentertained throughout. Perhaps they have simply not aged well. In the decades since this was written, others have written about similar ideas much more successfully, IMHO.
Both of the Mathematica stories appeared in Astounding Stories in 1936. They were reprinted in 2001 in The Best of John Russell Fearn Volume One edited by Philip Harbottle, which includes a nice cover painting by Ron Turner inspired by Mathematica.
Contributed by
Steve
I found both stories to be pretty ridiculous even accounting for when they were written. I have read nearly all of Astounding and Wonder before 1950 and have enjoyed almost all of it, but Fearn’s stories just infuriate me. Many of the readers writing to Brass Tacks (but not all) thought his work to be “drivel”. I agree. But I recognize others might disagree.

