SOLVED. YA Fantasy. 2 boys who don't like each other have to work together to find the disguised strands of Fenrir's(?) leash to prevent a war/ winter? Read in 1970's. [s]

[Marion]

I last read this back in the mid-to-late 70's. I remember one boy is bookish while the other is more physical (and I think was bullying the other boy?). I know they go from our world to another where magic works, and are promised a treasure if they can find (and recombine) the separate strands of the leash for Fenrir? (I recall a Norse/Ragnarok theme to the book).
They're given a magic rock that will flash when it touches one of the disguised strands and are sent out travelling Northwards(?) while they look. The other specific thing I remember was the bookish boy working on rhyme while they traveled, part of which went "the mathematics of the heart, brings together what's apart".
I've tried googling that line with no luck. Does this ring a bell with anyone

[Genesistrine]

The Hero from Otherwhere by Jay Williams

[Andy]

Yep. In "Hero from Otherwhere" I think the line is "The mathematics of the heart adds together what's apart" - so you were very close.

[Marion]

Genesistrine wrote: "The Hero from Otherwhere by Jay Williams"

Thanks for the rapid detective work!

[Marion]

Andy wrote: "Yep. In "Hero from Otherwhere" I think the line is "The mathematics of the heart adds together what's apart" - so you were very close."

Appreciate your help!

[Marion]

My search has been successful: the book I needed was "The Hero from Otherwhere" by Jay Williams.

Thanks for the community's help on this.