English Statistician is Positive to a Degree (5)

Across

9 ORPHANAGE (a school). The wordplay is an anagram (“isomorphism”) of ON A GRAPH followed by E, the base of the natural logarithm.

10 THETA. Cryptic definition as indicated by the question mark, not quite kosher. “The T. A.” does the grading, and THETA is a Greek letter.

11 PERFECT (= is better). Double definition; in graph theory, a forest is an example of a perfect graph (the definition-by-example is indicated by “maybe”).

12 ISRAELI (Alon, say). The wordplay is an anagram (“bizarre”) or AIRLINES without (“ignoring”) the letter N.

13 SETH (Vikram, Indian author). Double definition, the Strong Exponential Time Hypothesis in computational complexity speaks about the complexity of finding a satisfying assignment.

14 ASSIGNMENT (= job). ASS (“idiot”)  and MEN (“workers”) around an anagram (“cocktail”) of GIN followed by T (the first letter or “to”).

16 TOREROS (they fight). Section of “resTORE ROSamond.” Somewhat unreasonable definition balanced by a very easy clue.

17 HESSIAN (a matrix). The wordplay asks you to transform (i.e., find an anagram of) VANISHES and remove the V. I was tempted to let v tend towards epsilon instead (which means the empty string in formal language theory).

19 NONUNIFORM (what circuit complexity is). Homonym (“sounds like”) nun uniform, habit here means garment, not practice.

22 PAPA (= father). Acrostic (“originally”) of Parameterised And Polynomial-space Algorithms.

24 MINIMAL (= token, adj.). The two-player game NIM reversed (“about”) followed by an anagram (“encrypted”) of MAIL.

25 UNSTRAP (= free, adj.). Anagram of (“variable”) TURNS followed by the first letters (“leading”) of Proof Assistant reversed (“around”).

26 EXTRA (= additional). Second letters of “kEynote eXperiments sTymie bRilliant tAlk.”

27 OVERCOUNT (= compute |A ∪ B| as |A| + |B|, doubly counting the intersection). Wordplay is a cryptic (indicated by the question mark) way of describing Dracula’s ex, who after the break-up is soooo over him.

Down

1 COMPOSITE NUMBER (90, e.g.). Cryptic wordplay (indicated by ?) of a hammer (which makes you numb, so it’s a number) made of a composite material (such as carbon fibre).

2 OPERATOR (addition, e.g.). Charade of OPERA (of which lovely old Ludwig van wrote only one, Fidelio) followed by TOR (a functor in homological algebra).

3 BAYES (an English statistician). YES (“positive”) following B.A. (Bachelor of Arts, a degree).

4 BAPTISES (= introduces). Anagram (“problem”) of PIT inserted into BASES (linearly independent sets). Not quite perfectly constructed, because “P.I.T. problem” ought to be “P.I.T. problematically” or something like that for reasons of grammar.

5 MEDICI (a famous family of Renaissance Italy). MED (a degree) + ICI.

6 STAR ANISE (a tree). Anagram of (“decomposition of”) SATISNEAR. Very happy about this clue.

7 DELETE (= blank out). Middle letters from neroDELETErdös.

8 HAMILTONIAN PATH (a problem in the theory of algorithms). HA (“laugh”)  followed by an anagram of ANOPTIMALHINT. The anagram indicator “solving” is here in the meaning of solve = unfasten, loose.

15 TRINOMIAL (a² + b² + c² , e.g.). TRIAL (“challenge”) around MONI (Naor, Israeli computer scientist) reversed (“up”, because it’s a down clue).

17 HERCULES (who performed of many tasks). Anagram (“optimal”) of SCHEDULER without D (“spent day”).

18 IN A GROUP (= altogether). Double definition; in algebra, identity is unique in a group. I’m unhappy about the  surface reading of this clue.

20 NINETY (more than 64). Consecutive characters of “kilobYTE NINtendo”, reversed (“retro”).

21 FALLOW (= neglected). FLOW around first two letters (“starts”) of ALgorithm, reversed (“confusingly”).

23 PSYCH (threaten). Anagram of (“unruly”) SPY followed by CH, the Counting Hierarchy of Computational Complexity