Mathmech Sigma & Nabla & Addition & Subtraction
Full art:

Sigma:

“[Its colors are] more balanced than Substraction”

Mathmech Nabla DARK 4-stars:

Left: “A pattern similar to camouflage, looks a little bit like a mosaic. Because it becomes a Primathmech, it has an [altered/falsified] look”
Right: “Symmetric”

TL note: The word for “Primathmech” in JP is Kaizanki (塊斬機), with “Kaizan” being a pun on its homophone (改竄), meaning “alteration/falsification“. In other words, the reference art says Nabla looks different from the others precisely because of the pun.

Mathmech Multiplication & Division & Diameter & Circular
Full art:

Division:

“Type B”

Circular:

Left: “If needed, it can also become a sword”
Right: “Top view”

Thanks to our anonymous contributors for the scans.