Congruences after evaluation at 3/2
We use congruences modulo powers of 2 and 3 to find combinations of evaluated
polynomials with prescribed divisibility. The argument applies to
arbitrary integral polynomials, including the forms of [8]. The
denominator-clearing map must be the same for every row that will be
added. We therefore fix a common truncation index 𝑊 ≥0 and, for 𝑃(𝑋) =∑𝑖𝑝𝑖𝑋𝑖 ∈ℤ[𝑋], put
𝐻𝑊(𝑃)=𝑊∑𝑖=0𝑝𝑖3𝑖2𝑊−𝑖.
This is the denominator-cleared
evaluation at 3/2. Each term
𝑝𝑖𝑋𝑖 with 𝑖 ≤𝑊 contributes 𝑝𝑖3𝑖2𝑊−𝑖. Keep the index 𝑊 fixed when polynomials are added. When
𝑊 ≥deg𝑃, this is exactly the
cleared numerator, since
𝑊∑𝑖=0𝑝𝑖3𝑖2𝑊−𝑖=2𝑊𝑊∑𝑖=0𝑝𝑖(32)𝑖=2𝑊𝑃(32).
Here a unit coefficient in ℤ
means 1 or −1. Thus a monic polynomial of degree
𝑊 with constant term ±1 satisfies both unit conditions
below. Those conditions are sufficient, not necessary: the congruences
themselves only require that the relevant coefficient not be divisible
by the relevant prime.
Theorem 5.1 (endpoint residues). Let 𝑃 =∑𝑖𝑝𝑖𝑋𝑖 ∈ℤ[𝑋] and let 𝑊 ≥0. Then
𝐻𝑊(𝑃)≡𝑝02𝑊(mod3),𝐻𝑊(𝑃)≡𝑝𝑊3𝑊(mod2).
Consequently a unit constant
coefficient prevents divisibility by 3, and a unit coefficient at index 𝑊 prevents divisibility by 2.
Proof. Modulo 3, every
summand with 𝑖 >0 vanishes;
modulo 2, every summand with 𝑖 <𝑊 vanishes. The remaining powers are
units in the corresponding residue fields. ◻
Since 2𝑊 is invertible modulo
3 and 3𝑊 is invertible modulo 2, the two congruences say more than the
stated consequence: divisibility of 𝐻𝑊(𝑃) by 3 is decided by 𝑝0 alone, and divisibility by 2 by 𝑝𝑊 alone. The rest of the coefficient
vector is invisible to both primes. The coefficient 𝑝𝑊 is the top coefficient of 𝑃 exactly when deg𝑃 =𝑊, and is zero when deg𝑃 <𝑊. The identity 𝐻𝑊(𝑃) =2𝑊𝑃(3/2) is guaranteed when
deg𝑃 ≤𝑊. Without a degree
bound, increasing the truncation index gives
𝐻𝑊+1(𝑃)=2𝐻𝑊(𝑃)+𝑝𝑊+13𝑊+1,
so
the truncated sum doubles precisely when the extra coefficient is
zero.
Example 5.2. Take 𝑊 =2. The three polynomials below differ
only at an endpoint.
𝑃 |
𝐻2(𝑃) |
3 ∣𝐻2(𝑃) |
2 ∣𝐻2(𝑃) |
| 𝑋2 +1 |
1 ⋅4 +0 ⋅6 +1 ⋅9 =13 |
no |
no |
| 𝑋2 +3 |
3 ⋅4 +0 ⋅6 +1 ⋅9 =21 |
yes |
no |
| 2𝑋2 +1 |
1 ⋅4 +0 ⋅6 +2 ⋅9 =22 |
no |
yes |
The first has both endpoints equal to 1 and its evaluation, 13, is divisible by neither prime; as a
check, 22((3/2)2 +1) =13. The second
and third show that each hypothesis is used: spoiling the constant
endpoint admits 3, and spoiling the
top endpoint admits 2.
Formal proofs cover the congruences modulo
3 and modulo
2, together with the
consequences for a unit
constant coefficient and a unit
coefficient of 𝑋𝑊.
In the following proposition, a unit top endpoint means that the
coefficient at index 𝑊 is 1 or −1, with no degree bound imposed. The two
conditions apply to different entries of the polynomial pair.
Proposition 5.3 (common divisor). Let 𝑈,𝑉 ∈ℤ[𝑋] and let 𝑊 ≥0. If 𝑈 has unit top endpoint, 𝑉 has unit constant endpoint, and an
integer 𝑐 divides both 𝐻𝑊(𝑈) and 𝐻𝑊(𝑉), then
2∤𝑐and3∤𝑐.
Proof. If 2 ∣𝑐, then
2 ∣𝐻𝑊(𝑈), contrary to the unit
coefficient at index 𝑊 and
Theorem 5.1. Similarly,
3 ∣𝑐 would imply 3 ∣𝐻𝑊(𝑉), contrary to the unit
constant coefficient. ◻
Lean checks this common-divisor
exclusion. The proposition does not say that the two evaluations are
coprime; it says only that every common divisor is coprime to 6. It gives no restriction on the other
prime factors of that divisor.
Example 5.4. Take 𝑊 =2, 𝑈 =𝑋2 +3 and 𝑉 =5𝑋2 +1. The top endpoint of 𝑈 and the constant endpoint of 𝑉 are both 1, and
𝐻2(𝑈)=3⋅4+1⋅9=21,𝐻2(𝑉)=1⋅4+5⋅9=49.
Here gcd(21,49) =7. The endpoint assumptions
therefore allow a nontrivial common divisor, but that divisor is coprime
to 6.
Corollary 5.5 (limits of rescaling and common-divisor
cancellation at 3/2). Under
the endpoint hypotheses of Proposition 5.3, every
common divisor of the unscaled evaluations 𝐻𝑊(𝑈) and 𝐻𝑊(𝑉) is coprime to 6. Multiplying two integer rows by
nonzero integers 𝑐𝑛,𝑐𝑚 multiplies
their determinant by 𝑐𝑛𝑐𝑚 and its
absolute value by |𝑐𝑛𝑐𝑚|.
Cancelling this introduced scalar factor therefore leaves the original
comparison between divisor and determinant size unchanged.
Proof. The common-divisor assertion is Proposition 5.3. By
Theorem 4.1,
rescaling the two rows by 𝑐𝑛,𝑐𝑚
multiplies their errors by 𝑐𝑛,𝑐𝑚,
respectively, and their 2 ×2
determinant by 𝑐𝑛𝑐𝑚. For nonzero
scalars, the introduced divisor has the same absolute value as the
introduced factor in that determinant. Dividing out that factor restores
the original determinant, including its original divisor-to-size
comparison. ◻
One further consequence of Theorem 5.1 is worth
stating, because it bears on the most natural way one might hope to
import an existing denominator reduction. Write Φ𝑚 for the 𝑚th cyclotomic polynomial and, for
coprime 𝑎 >𝑏 ≥1, put Φ𝑚(𝑎,𝑏) =𝑏𝜑(𝑚)Φ𝑚(𝑎/𝑏) for
its homogenisation, with exponent 𝜑(𝑚) =degΦ𝑚.
Proposition 5.6 (coprimality of homogenised
cyclotomic values). Let 𝑎 >𝑏 ≥1 with gcd(𝑎,𝑏) =1 and let 𝑚 ≥1. Then gcd(Φ𝑚(𝑎,𝑏),𝑎𝑏) =1. In particular
gcd(Φ𝑚(3,2),6) =1 for every
𝑚.
Proof. The polynomial Φ𝑚 is monic and Φ𝑚(0) = ±1. The endpoint argument in
Theorem 5.1 also works
at (𝑎,𝑏): modulo a prime dividing
𝑏, only 𝑎𝜑(𝑚) survives, and modulo a
prime dividing 𝑎, only Φ𝑚(0)𝑏𝜑(𝑚) survives.
Coprimality of 𝑎 and 𝑏 makes each surviving term
nonzero. ◻
Rhin and Viola’s factorial-coset quotients [17] reduce denominators of rational forms
in 𝜁(2). Zudilin’s 𝑞-analogue has an order-twelve symmetry
group for the normalised series; its denominator cancellation uses the
order-six subgroup preserving the required sign condition [8]. Proposition 5.6 applies to
these cyclotomic factors: each homogenised value at (3,2), and hence any product of them, is
divisible by neither 2 nor 3. It makes no such assertion about an
arbitrary integer or factorial factor, which can contain both primes.
Cancelling a common cyclotomic factor can still reduce the real absolute
values. That reduction is distinct from producing the powers of 2 or 3 sought in Section 10.
Scalar multiplication can introduce powers of 2 and 3, but does not improve this comparison.
Under the endpoint hypotheses, the unscaled pair has no common factor at
either prime. Neither observation shows that a gain at these primes is
necessary for a proof by linear forms at 3/2.
The operation studied next is different: take an integer combination
of several rows and ask that the combination be divisible where the
individual rows are not. The proposed divisor must come from
cancellation between rows, not from multiplying a row by that divisor.
The endpoint congruences are the first case of a divisibility condition
that can be imposed to any depth, and it is that condition, read
additively, which is counted below.
We first raise the two congruences to prime powers. Fix depths 𝑅,𝑆 ≥0. For 𝑃 ∈ℤ[𝑋], let 𝐽3,𝑅(𝑃) and 𝐽2,𝑆(𝑃) be the residues of 𝐻𝑊(𝑃) modulo 3𝑅 and 2𝑆, respectively. Theorem 5.1 computes the
depth-one residues. At depth 𝑅,
terms with 𝑖 ≥𝑅 are invisible
modulo 3𝑅; at depth 𝑆, terms with 𝑊 −𝑖 ≥𝑆 are invisible modulo 2𝑆. Thus deeper congruences inspect
longer segments of the two ends of the coefficient list, always at the
same fixed 𝑊.
Lemma 5.7 (a residue and divisibility by 3𝑅). Vanishing of the residue
modulo 3𝑅 is exactly
divisibility by the corresponding power of three: 𝐽3,𝑅(𝑃) =0 if and only if 3𝑅 ∣𝐻𝑊(𝑃).
Proof. By definition 𝐽3,𝑅(𝑃) is the image of the integer
𝐻𝑊(𝑃) in ℤ/3𝑅ℤ. The kernel of
this quotient map is 3𝑅ℤ,
which proves the equivalence, including 𝑅 =0. ◻
The same argument gives 𝐽2,𝑆(𝑃) =0 ⟺ 2𝑆 ∣𝐻𝑊(𝑃) for 𝑆 ≥0.
The vector of four residues of a coefficient pair (𝑈,𝑉) is then the quadruple
(𝐽3,𝑅(𝑈),𝐽3,𝑅(𝑉),𝐽2,𝑆(𝑈),𝐽2,𝑆(𝑉))∈(ℤ/3𝑅ℤ)2×(ℤ/2𝑆ℤ)2,
two residues for each of
the two primes, one from each entry of the pair. By the displayed
criteria it vanishes exactly when 3𝑅 divides both specialised entries and
2𝑆 divides both.
Instead of requiring each input row to have a common divisor, we seek
a small integer combination whose two entries are both divisible by
3𝑅2𝑆. Since 𝐻𝑊 is additive, the four residues of the
combination are the corresponding sums of the input residues. This
allows a pigeonhole argument on subset sums.
Theorem 5.8 (equal residues for two subset sums).
Fix a truncation index 𝑊 and
depths 𝑅,𝑆, and let (𝑈𝑗,𝑉𝑗)𝑗<𝑀 be any 𝑀 pairs of integral polynomials.
Represent each subset of {0,…,𝑀 −1} by its indicator vector
in {0,1}𝑀. If the 2𝑀 subsets outnumber the possible
residue vectors in
(ℤ/3𝑅ℤ)2×(ℤ/2𝑆ℤ)2,
then two
distinct subsets have the same residue vector. Subtracting their
indicator vectors gives a nonzero coefficient vector in { −1,0,1}𝑀 cancelling all four
residues. The target has exact cardinality
(3𝑅)2(2𝑆)2.
In particular, if 𝑅 >0 and 4𝑅 +2𝑆 ≤𝑀, such a collision
exists.
Proof. Send each subset to the sum of the residue vectors of
its members. The pigeonhole principle gives two distinct subsets with
the same residue vector. The number of possible vectors is the product
of the four moduli. For 𝑅 >0,
(3𝑅)2(2𝑆)2<(4𝑅)2(2𝑆)2=24𝑅+2𝑆≤2𝑀,
which proves the stated sufficient threshold. ◻
The power bracket improves the generic coefficient 4𝑅 when the depth 𝑅 is a positive integer multiple of 41. All depths and row counts in the
following corollary, including 𝑇,
are integers.
Corollary 5.9 (the exact count at depth 41). Let 𝑇 >0. At modulus 3𝑅 with 𝑅 =41𝑇, any family of 𝑀 ≥130𝑇 +2𝑆 integral polynomial pairs has
two distinct binary vectors with the same residue vector. For 𝑇 =1 the coefficient 130 is exact for this counting argument:
2129+2𝑆<|
|
|(ℤ/341ℤ)2×(ℤ/2𝑆ℤ)2|
|
|.
No exact-optimality
assertion is made here for 𝑇 >1.
Proof. The upper power inequality gives
(341𝑇)2(2𝑆)2<(265)2𝑇(2𝑆)2=2130𝑇+2𝑆≤2𝑀,
so Theorem 5.8 applies. For
𝑇 =1, direct integer evaluation
gives 2129 <382;
multiplying by (2𝑆)2 gives the
displayed reverse count at 129 +2𝑆. ◻
The formal statements check the sufficient
count 130𝑇 +2𝑆 and, when 𝑇 =1, the failure
of the count at 129 +2𝑆.
For all depths at once the ambient-cardinality inequality 2𝑀 >32𝑅22𝑆 holds exactly when
𝑀 ≥ ⌊2𝑅log23+2𝑆⌋+1,
by the definition of
the floor function; irrationality of log23 is not needed for this strict-inequality reformulation. At
𝑅 =41 this is 130 +2𝑆, and at 𝑅 =41 ⋅31 it is 4029 +2𝑆, one below the uniform bound
4030 +2𝑆 of Corollary 5.9; the
corollary trades exactness for a certificate that is a single integer
comparison. Here 41 is the exponent
of 3 in the modulus, not the number
of input rows; at that depth the counting bound is 130 +2𝑆 rows.
The count produces two distinct subsets, not necessarily two distinct
row sums or remainder values. Repeated rows already illustrate the first
failure. To prevent the second, we bound how many subsets can have any
specified remainder. The following finite-set statement isolates that
requirement.
Theorem 5.10 (equal residues with different values).
Let 𝐴 and 𝐵 be finite sets, let 𝑓 :𝐴 →𝐵, and let 𝑔 :𝐴 →𝐶 be any map into a set 𝐶. Suppose every fibre of 𝑔 has at most 𝑘 elements. If
|𝐵|𝑘<|𝐴|,
then there exist distinct
𝑥,𝑦 ∈𝐴 such that
𝑓(𝑥)=𝑓(𝑦)and𝑔(𝑥)≠𝑔(𝑦).
Thus, when 𝑓 records
the four residues and 𝑔 records the
real remainder, a bound on the multiplicities of equal remainders
guarantees a pair with equal residues and different remainders.
Proof. If every pair in a common 𝑓-fibre also had the same 𝑔-value, each 𝑓-fibre would lie in one 𝑔-fibre and hence have size at most 𝑘. Summing over the at most |𝐵| fibres of 𝑓 would give |𝐴| ≤|𝐵|𝑘, contrary to the
hypothesis. ◻
If 𝑔 is injective, the
hypothesis holds with 𝑘 =1. It need
not hold with a useful small 𝑘 for
subset sums: repeated rows produce many equal sums, even when every row
is primitive. The theorem assumes a bound on every fibre of 𝑔, not merely on the fibres of (𝑓,𝑔). It gives a nonzero difference but
no upper bound on its real size. To bound the size as well, we partition
the range of 𝑔 into small
intervals. The count now uses residue class, interval and multiplicity.
Only repetitions within one residue class matter, so the quantitative
version assumes a bound for fibres of (𝑓,𝑔) rather than for all fibres of 𝑔.
Theorem 5.11 (equal residues with close, distinct
values). Let 𝐴,𝐵,𝐽 be finite
sets and let 𝑓 :𝐴 →𝐵, 𝑔 :𝐴 →ℝ and 𝜄 :𝐴 →𝐽. Suppose that each
simultaneous fibre of (𝑓,𝑔) has at
most 𝑘 elements and that, for some
𝛿 >0,
𝜄(𝑥)=𝜄(𝑦)⟹|𝑔(𝑥)−𝑔(𝑦)|<𝛿.
If |𝐵||𝐽|𝑘 <|𝐴|, then some distinct 𝑥,𝑦 ∈𝐴 satisfy
𝑓(𝑥)=𝑓(𝑦),0<|𝑔(𝑥)−𝑔(𝑦)|<𝛿.
Proof. Suppose that no such pair exists. Two points in a
fibre of (𝑓,𝜄) have equal 𝑓-values and 𝑔-values less than 𝛿 apart, so their 𝑔-values must be equal. Thus each fibre
of (𝑓,𝜄) lies in a fibre of
(𝑓,𝑔) and contains at most 𝑘 points. Summing over its at most |𝐵||𝐽| fibres gives |𝐴| ≤|𝐵||𝐽|𝑘, a contradiction. ◻
For an application, let 𝑀
integral coefficient rows (𝐴𝑗,𝐵𝑗)
have real remainders 𝑒𝑗 at the
same target. Let 𝐷 be a positive
integer, and suppose that the coefficient-pair subset sums ∑𝑗∈𝐼(𝐴𝑗,𝐵𝑗) take 𝑄 values in (ℤ/𝐷ℤ)2. Put 𝑇 =∑𝑗|𝑒𝑗|. The least and greatest
subset remainders are respectively
∑𝑒𝑗<0𝑒𝑗and∑𝑒𝑗>0𝑒𝑗,
so the interval between them has length 𝑇. Divide it into half-open intervals of
width 𝐷/𝑛, using a final interval
for the right endpoint if necessary. At most ⌊𝑛𝑇/𝐷⌋ +1 intervals are
needed. If each simultaneous residue-and-remainder fibre has size at
most 𝑘, the inequality
2𝑀>𝑄𝑘(⌊𝑛𝑇/𝐷⌋+1)
therefore gives equal residues with
remainder difference strictly between 0 and 𝐷/𝑛 in absolute value. Subtracting the
two subset sums and dividing both coefficients by 𝐷 gives an integral form of nonzero
absolute value less than 1/𝑛. For a
prescribed family, the number 𝑄,
the width 𝑇 and the multiplicity
bound 𝑘 must all be justified. We
have no such bounds giving the required forms for the Zudilin family at
3/2.
The next improvement needs a much stronger hypothesis than
primitivity: adjacent determinants must vanish modulo the chosen
modulus. Unit multiples of one unimodular row satisfy it. Arbitrary
primitive rows do not; for example, (1,0) and (0,1) have determinant 1. Under this hypothesis the rows lie in
the free rank-one submodule generated by a unimodular row. A change of
coordinates makes their second coordinates zero, so only one residue
coordinate must be counted.
Theorem 5.12 (vanishing minors and a residue count).
Let 𝑅0 be a commutative ring
and let 𝑤𝑛 =(𝐴𝑛,𝐵𝑛) ∈𝑅20.
Suppose that each row is unimodular, meaning that 𝑢𝑛𝐴𝑛 +𝑣𝑛𝐵𝑛 =1 for some 𝑢𝑛,𝑣𝑛 ∈𝑅0, and that every adjacent
minor vanishes:
𝐴𝑛𝐵𝑛+1−𝐵𝑛𝐴𝑛+1=0(𝑛≥0).
Then every pairwise minor 𝐴𝑖𝐵𝑗 −𝐵𝑖𝐴𝑗 vanishes. In particular,
take 𝑅0 =ℤ/(2𝑆3𝑅)ℤ with 𝑅 >0. If 𝑆 +2𝑅 ≤𝑘, there are two distinct binary
vectors 𝑠,𝑡 ∈{0,1}𝑘 such that
∑𝑖<𝑘𝑠𝑖𝑤𝑖=∑𝑖<𝑘𝑡𝑖𝑤𝑖.
Thus 𝑆 +2𝑅 rows suffice. The ambient
two-coordinate argument gives the sufficient bound 2𝑆 +4𝑅.
Proof. If (𝑎,𝑏) is
unimodular, say 𝑢𝑎 +𝑣𝑏 =1, and 𝑎𝑦 −𝑏𝑥 =0, then
(𝑥,𝑦)=(𝑢𝑥+𝑣𝑦)(𝑎,𝑏).
Indeed, the first
coordinate follows by replacing 𝑎𝑦
with 𝑏𝑥, and the second by the
reverse substitution. Apply this identity to consecutive rows: each next
row is a scalar multiple of the current one. Induction places the entire
tail on the line through 𝑤0 and
proves the pairwise-minor assertion. Right multiplication by
(𝑢0−𝐵0𝑣0𝐴0)
has determinant 𝑢0𝐴0 +𝑣0𝐵0 =1 and
sends 𝑤0 to (1,0). All transformed subset sums
therefore have second coordinate zero and occupy at most 2𝑆3𝑅 values. Finally
2𝑆3𝑅<2𝑆4𝑅=2𝑆+2𝑅≤2𝑘,
so the
pigeonhole principle gives two distinct subsets with the same sum. ◻
No particular coordinate needs to be invertible: modulo six, (2,3) is unimodular because −2 +3 =1, although neither entry is a unit.
Some nondegeneracy is essential. The rows (1,0),(0,0),(0,1) have zero adjacent
minors but outer minor one. For the counting conclusion, vanishing is
required only in the quotient ring. The integer rows (1,0),(1,6) used in Section 10 are independent
over ℚ, with determinant 6, but coincide modulo 6. Thus dependence modulo the modulus
neither follows from primitivity nor implies dependence of the integer
rows.
This conditional theorem is stronger than the ambient count of
possible residue vectors only after its minor-vanishing hypothesis has
been established. No such all-tail hypothesis is proved here for an
actual 𝑞-Apéry or Zudilin family,
and the theorem says nothing about whether the difference of the subset
sums has nonzero analytic remainder.
The count in Corollary 5.9 is sharp
at 𝑇 =1 for comparison with the full
residue space. To obtain different remainders, Theorem 5.10
additionally needs a bound on how often the same real value occurs. Such
a bound is not proved here for the 𝑞-Apéry or Zudilin remainder family.
Lean checks the cardinality
of the residue space, the pigeonhole
argument for the residue map and the sufficient
row count for equal residues. The pigeonhole argument gives
divisibility without an independence hypothesis. Further information
about the family is needed to ensure that the nonzero difference of the
indicator vectors gives a nonzero polynomial pair and a nonzero analytic
remainder. None of these statements supplies such a family or proves
either nonvanishing conclusion.
Example 5.13. At depths 𝑅 =𝑆 =1 the space of four residue
coordinates is (ℤ/3ℤ)2 ×(ℤ/2ℤ)2, of
cardinality 9 ⋅4 =36, and the
threshold reads 𝑀 ≥4 ⋅1 +2 ⋅1 =6. With six pairs
there are 26 =64 subsets and only
36 possible residue vectors. Two
subsets therefore have equal residues. The difference of their indicator
vectors is a nonzero vector in { −1,0,1}6 whose sum vanishes in all
four coordinates.
The following elementary comparison concerns only the two scalar
exponents of Zudilin’s construction; it makes no assertion about the
coefficient polynomials or their real remainders.
Theorem 5.14 (a restriction on the two scalar
exponents). Let 𝐶1 >0. If
𝐶0 ≤0 or 2𝐶0 ≤𝐶1, then
𝐶0log3−𝐶1log2<0.
In the
positive branch 𝐶0 >0 and 2𝐶0 ≤𝐶1, the stronger estimate is
𝐶0log3−𝐶1log2<−1741𝐶0log2.
Proof. If 𝐶0 ≤0, then
𝐶0log3 −𝐶1log2 ≤ −𝐶1log2 <0.
If 𝐶0 >0 and 2𝐶0 ≤𝐶1, use 341 <265 to obtain
𝐶0log3−𝐶1log2≤𝐶0(log3−2log2)<−1741𝐶0log2<0.
◻
Written multiplicatively, the conclusion is 3𝐶0 <2𝐶1. Lean checks the inequality
and the 17/41 margin of the positive branch.
The scope of this elementary inequality matters. The primary Zudilin
theorem [8] supplies
an integer-base irrationality-exponent estimate on its parameter cone,
and the elementary inequality 𝜇 ≥2 then forces 2𝐶0 ≤𝐶1 whenever 𝐶0 >0; Lean checks that implication
separately. The primary 2004 theorem is stated for an integer inverse
base. The rational specialisation proved earlier in this record has
separate proofs for the constructed forms. The scalar statement in this
paragraph is only the displayed inequality; it neither constructs a new
family at 3/2 nor formalises a
universal exclusion of linear-form methods.