Center of skew rings and kernel of RingMap to skew rings - #4485
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joel-dodge wants to merge 6 commits into
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Center of skew rings and kernel of RingMap to skew rings#4485joel-dodge wants to merge 6 commits into
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joel-dodge
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July 31, 2026 17:53
d-torrance
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bugs/dan/1-jacobian through 1-loadPackage. Six fixed, two duplicates, two wontfix, two filed. Filed: * Macaulay2#4589 1-join-hash -- the guess in the file is right and the saving is real. Every List construction folds the whole list (basic.d:114-117 does r.hash = hash(r); the fold is at :82-85), so join of two 200k lists costs 11.2ms of which 7.5-8.5ms is hashing -- measurable because a Sequence's hash is not cached and refolds on demand. The fold is combinable: h(L++M) = 1299833^#M * (hash L - c) + hash M mod 2^64, c = hash class L + 23407, verified on seven cases. My first attempt used the Sequence constants and failed all five, which is how the two folds turned out to differ. * Macaulay2#4590 1-kernel-ringmap -- Mike's 2008 recipe, unevenly landed. The Hilbert hint is implemented for degree-preserving maps (ringmap.m2:320-332, verified live: gb records rawGBSetHilbertFunction log), the inhomogeneous case it was written for is skipped because canUseHilbertHint requires isHomogeneous (gb.m2:271), and the recipe's forceGB step is Macaulay2#686. PR Macaulay2#4485 is adding another strategy to the same table. Duplicates, both with comments posted: 1-kernel-ringmap-ZZ -> Macaulay2#1518 (the message moved to methods.m2:695, and ringmap.m2:336's ZZ strategy only fires when the source is ZZ itself), 1-line-wrapping-with-negative-numbers -> Macaulay2#4149 (the same splitcolumn gap one character class over; three lines still end in a bare hyphen). Fixed: 1-leadTerm-crash.m2 (the ignored engine error is propagated, so enginering.m2:496 errors instead of smoking), 1-lift-RR-CC (both spellings return precision 53), 1-lift-modules (lift((QQ[x])^3, QQ) is QQ^3), 1-lifting (.3 * id_(QQ^3) is over RR_53 now, and promote(.3, QQ) errors -- the direction PR Macaulay2#62 on MichaelABurr/M2 is formalising), 1-lcm-needed (implemented, gcd compiled at 14.3us on 700-digit inputs, and the Binary ask declined on measurement: 9.7ms through Binary against 13.8ms for a hand fold), 1-loadPackage (PR Macaulay2#3852's warning meets the need; the file's own example is invalid today for a rank reason and fails identically with no reload). Wontfix: 1-jacobian (the intrinsic object is one line away, and jacobian-doc documents the present behaviour as "identical to jacobian generators I"), 1-list-of-ideals-of-R (asked and declined in PR Macaulay2#3328). dan/1 todo 154 -> 142, settled 351 -> 363.
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Add
centerRingmethod to compute the center of a ring. Currently supports skew polynomial rings (and commutative rings 😂 ).Then adds a new strategy for computing the kernel of a RingMap whose target is SkewCommutative by reducing to a map to the center of the target.
This requires some awkward packaging since
kernelis in the core and the strategy for computing the center of a skew commutative ring depends on the PushForward package. We rely here on the "hack" of callingneedsPackageinside the hook only when needed although @d-torrance says "I'm not sure if this is a very elegant solution" and I am inclined to agree.Note while editing ringmap.m2 I took the opportunity to clean up ws formatting in the block declaring different kernel strategies for RingMap. Consider toggling the "hide whitespace" option while reviewing this PR.