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ERDőS #261 · PARTIAL

Erdős problem 261 — wave 8p report

Date of live-page access and computation: 2026-07-28 (UTC).

Claim labels used throughout:

0. Mandatory live-page gate

(a, direct live-page transcription) I fetched

the live problem page, its

LaTeX view, and its

discussion thread through

the Bright Data browser path. The page was last edited 2025-12-01 and showed:

There was therefore no collision/skip condition.

(a, direct live-page transcription) The source line is

#261: [Er74b][ErGr80][Er88c,p.104], and the displayed tag is

number theory.

Verbatim current statement (copied from the live LaTeX view):

> Are there infinitely many $n$ such that there exists some $t\geq 2$ and distinct integers $a_1,\ldots,a_t\geq 1$ such that\[\frac{n}{2^n}=\sum_{1\leq k\leq t}\frac{a_k}{2^{a_k}}?\]Is this true for all $n$? Is there a rational $x$ such that\[x = \sum_{k=1}^\infty \frac{a_k}{2^{a_k}}\]has at least $2^{\aleph_0}$ solutions?

(b, live-page cited results) The page lists the following established

context.

1. The problem is related to problem 260.

2. Erdős reported that Cusick had a simple proof of infinitely many admissible

\(n\), but did not record it.

3. Borwein and Loring gave the explicit identity

\[ n=2^{m+1}-m-2,\qquad \frac n{2^n}=\sum_{n

for every positive integer \(m\). For the stated \(t\ge2\) condition take

\(m\ge2\).

4. Tengely, Ulas, and Zygadło verified the required property for

\(n\le10{,}000\).

5. Erdős's 1988 formulation asks the weaker-looking question of a rational

\(x\) having two representations.

(a, independent check of (BL)) Clearing \(2^{n+m}\), the right side is

\[ \sum_{i=1}^{m}(n+i)2^{m-i} =n(2^m-1)+(2^{m+1}-m-2)=n2^m, \]

which is the cleared left side. Also

\(n_{m+1}-n_m=2^{m+1}-1>0\), so this supplies infinitely many distinct

\(n\). The checker independently verifies this identity for

\(2\le m\le200\).

All three live comments

(a, exact arithmetic; comment itself is not treated as peer review) Vjeko

Kovac's 2025-08-14 comment says that the \(a_i\) must be mutually distinct,

corrects an earlier attribution of (BL) to himself in favor of Borwein--Loring

while retaining credit for Cusick, and says that the all-\(n\) question remains

open with \(n\le10{,}000\) checked. The site says it was updated in response.

(a, exact arithmetic; comment itself is not treated as peer review)

Zeraoulia Rafik's 2026-04-27 comment observes

\[ \frac4{2^4}=\frac5{2^5}+\frac6{2^6}=\frac14,\qquad \sum_{m\ge1}\frac m{2^m}=2. \]

Thus, under the literal infinite-subset interpretation,

\[ \frac74 =\sum_{a\in\mathbb N\setminus\{5,6\}}\frac a{2^a} =\sum_{b\in\mathbb N\setminus\{4\}}\frac b{2^b}. \]

This correctly gives two representations, but not continuum many.

(c, reported opinion only) Vjeko Kovac's 2026-05-05 comment says it is

generally believed that “two” in the weakened formulation was a typo or

misreading for \(2^{\aleph_0}\). This is historical interpretation, not a

mathematical result.

The live thread warns that comments are user-provided and unverified; I have

accordingly used only the displayed elementary identities, not the comments as

authority for open status.

1. Primary-source and later-literature audit

(b) Erdős's primary 1988 paper,

“On the irrationality of certain series: problems and results”,

pp. 102--109, states the finite equation on p. 104, asks “infinitely many

\(n\), or perhaps all \(n\)?”, and asks for a rational \(x\) with two

representations. Its footnote says Cusick communicated a proof of the

infinitely-many assertion in June 1987 and that the all-\(n\) question remained

open.

(b) Borwein and Loring's primary paper,

[“Some questions of Erdős and Graham on numbers of the form

\(\sum g_n/2^{g_n}\)”](https://www.cecm.sfu.ca/~pborwein/PAPERS/P46.pdf),

Math. Comp. 54 (1990), 377--394, proves (BL). It formulates its stronger

Conjecture 1 that every integer-initialized orbit

\[ e_{j+1}=2(e_j\bmod j) \tag{BLC} \]

eventually reaches zero. It also proves that a dense set of irrational

numbers has continuum many star-binary representations. Its rational

consequences are conditional or give arbitrarily large finite multiplicity,

not the requested rational continuum fiber.

(b) Tengely--Ulas--Zygadło,

“On a Diophantine equation of Erdős and Graham”,

J. Number Theory 217 (2020), 445--459

(arXiv:2008.01501), proves bounds for fixed

term count, enumerates solutions through \(k=8\), constructs high-multiplicity

families, gives rational numbers with at least nine representations, and uses

the exact greedy recurrence to verify \(n\le10^4\). It explicitly leaves the

all-\(n\) question open.

(d, search audit rather than an exhaustiveness theorem) Exact-title,

exact-equation, author, DOI-citation, arXiv, Crossref, OpenAlex, and Semantic

Scholar searches on 2026-07-28 found no later primary paper extending the

\(10^4\) range or resolving either live question. The three citation indices

each reported one citation of TUZ20: Hančl--Luca--Tachiya,

[“Irrationality and linear independence results for certain infinite series

related to \(r\)-full numbers”](https://doi.org/10.1007/s10998-025-00696-4)

(2026). Its publisher abstract concerns irrationality of series supported on

\(r\)-full numbers, not the finite representation equation here. Citation

indices are incomplete, so this is an honest search result, not proof that no

uncatalogued literature exists.

2. Exact reduction to one integer orbit

Fix \(n\ge2\). Scan candidate exponents \(a=n+1,n+2,\ldots\) in increasing

order, greedily taking \(a\) whenever its term fits in the residual.

(a) Before examining \(a\), let

\[ R_a=\frac n{2^n} -\sum_{\substack{bThen \(r_{n+1}=n\). The condition that \(a/2^a\) fits is exactly

\(a\le2r_a\), and clearing the next power of two gives

\[ r_{a+1}= \begin{cases} 2r_a-a,&2r_a\ge a,\\ 2r_a,&2r_aSince \(0\le r_{n+1} \[ 0\le r_awhere the remainder lies in \(\{0,\ldots,a-1\}\).

(a) If the orbit first reaches \(r_{A+1}=0\), the selected exponents form a

finite, strictly increasing set and

\[ n\,2^{A-n}=\sum_{a\ {\rm selected}}a\,2^{A-a}. \tag{3} \]

Thus they are an exact witness to the original equation. The first candidate

\(n+1\) is selected, after which the residual state is \(n-1>0\); hence every

terminating orbit for \(n\ge2\) selects at least two terms. No floating point

or tolerance enters (1)--(3).

(a) A nonzero orbit can terminate at exponent \(a\) only when \(a\) is even

and \(r_a=a/2\). Indeed, \(0

\(2r_a=a\).

(a) On putting \(e_a=2r_a\), (1) becomes

\[ e_{a+1}=2(e_a\bmod a), \]

which is precisely the Borwein--Loring recurrence (BLC), here on the special

initial data \(e_{n+1}=2n\). Consequently, the exact missing sufficient lemma

for the greedy route is:

> For every \(n\ge2\), the orbit \(r_{n+1}=n\),

> \(r_{a+1}=(2r_a)\bmod a\) reaches zero.

Proving this diagonal case would settle the all-\(n\) question affirmatively.

The full Borwein--Loring conjecture is stronger because it permits arbitrary

integer initial data. Conversely, failure of one greedy orbit would not

disprove the original problem, since a non-greedy finite representation might

still exist.

3. New exact finite computation

(d) The recurrence (1) terminates for every

\[ \boxed{1\le n\le100{,}000}. \]

For \(n=1\), where the greedy one-term equality is inadmissible because

\(t\ge2\), the separately checked witness is

\[ \frac12=\frac3{2^3}+\frac6{2^6}+\frac8{2^8}. \]

For every \(2\le n\le100{,}000\), the terminating greedy orbit supplies the

required distinct exponents. This extends the primary-source range

\(n\le10{,}000\) by a factor of ten.

(d) Aggregate certificate for \(2\le n\le100{,}000\):

| quantity | exact value |

|---|---:|

| total recurrence updates \(\sum(A(n)-n)\) | 17,929,666,741 |

| total selected terms \(\sum k(n)\) | 8,946,326,760 |

| FNV-1a digest of all ordered triples \((n,k(n),A(n))\) | 9f0a3e00dd8f0008 |

| largest term count | 285,549 at \(n=88,225\) |

| largest terminal exponent | 659,488 at \(n=88,225\) |

Here \(A(n)\) is the final selected exponent. The digest serializes each of

\(n,k(n),A(n)\) as eight little-endian bytes, in increasing order of \(n\).

(d) The simultaneous record holders are:

| \(n\) | number of terms \(k(n)\) | final exponent \(A(n)\) |

|---:|---:|---:|

| 2 | 3 | 8 |

| 5 | 5 | 14 |

| 7 | 7 | 24 |

| 8 | 13 | 32 |

| 10 | 189 | 392 |

| 56 | 6,092 | 12,230 |

| 3,113 | 13,370 | 29,752 |

| 3,817 | 76,072 | 155,942 |

| 5,325 | 227,040 | 460,536 |

| 88,225 | 285,549 | 659,488 |

Correction to TUZ20 Table 2

(a) The published last row

\((n,k(n),a_k(n))=(5588,460536,226913)\) cannot be correct as printed:

460,536 distinct exponents all greater than 5,588 cannot fit below 226,913,

since there are only \(226913-5588=221325\) available integers.

(d) Exact recurrence and cleared-denominator recomputation give instead

\[ (n,k(n),A(n))=(5588,226913,460536). \]

Thus the last two entries were swapped. There is a second consequence:

\(n=5588\) is not the term-count record claimed by that table, because the

earlier \(n=5325\) has 227,040 terms and the same final exponent 460,536.

(d) The standalone checker reconstructs every listed record witness and

the \(n=5588\) witness in Python, then independently checks (3) as an exact

big-integer equality. The two largest highlighted checks have cleared integers

of 455,224 bits (\(n=5325\)) and 571,280 bits (\(n=88225\)), respectively.

4. Reproduction

The entire mathematical kernel is:

def greedy_terms(n):
    r, a, terms = n, n + 1, []
    while r:
        r *= 2
        if r >= a:
            r -= a
            terms.append(a)
        assert 0 <= r < a
        a += 1
    return terms

def exact_check(n, terms):
    A = terms[-1]
    assert n << (A - n) == sum(a << (A - a) for a in terms)

The standalone checker is

erdos261_wave8p_verify.py.

The transparent serial discovery implementation is

erdos261_sweep.c.

Run:

python runs/erdos261_wave8p_verify.py

Observed output on this VM:

PASS: exact greedy witnesses for every 1 <= n <= 100000
total recurrence steps (n>=2): 17929666741
total selected terms (n>=2):   8946326760
FNV-1a certificate:             9f0a3e00dd8f0008
record holder:                  (88225, 285549, 659488)
TUZ20 corrected row:            (5588, 226913, 460536)
elapsed wall time:               17.528 s

(a) The C worker embedded in the checker uses unsigned 64-bit integers, but

the invariant \(0\le r_a

impossible in this run. Each job has a five-billion-update cap; no orbit came

remotely close to it. The checker then uses a separately written Python

implementation and arbitrary-precision integers for the displayed exact

identities.

5. What remains, precisely

(a) No finite computation, including \(10^5\), supplies the quantifier

“for every \(n\).” The exact outstanding lemma for this approach is uniform

termination of the diagonal orbit in Section 2. Current elementary facts

\(0\le r_a

force such a hit. The record jumps from \(A(3817)=155942\) to

\(A(5325)=460536\) and then \(A(88225)=659488\), illustrating why a bound

in terms of the checked range was not inferred.

(c, cost estimate only) A naive compiled sweep to \(10^6\), if the present

average cost merely scaled, would require roughly \(1.8\times10^{11}\) state

updates, about 0.2 core-hour at the measured \(2.1\)--\(2.6\times10^8\)

updates/second (well under \$0.05 at a typical \$0.05--\$0.10/core-hour

commodity rate). Heavy-tail record orbits could make it substantially larger.

Such a sweep would still not address the uniformity lemma, so I did not spend

that computation.

(b) For the continuum question, Borwein--Loring already obtain independent

finite replacements from (BL), hence continuum many representations of

certain explicitly defined numbers \(B_M\); they prove those \(B_M\) are

irrational. TUZ20 obtains infinitely many rational numbers with at least nine

representations, still finite multiplicity.

(a) The precise constructive gap is therefore not “find another finite

identity.” One needs infinitely many independent choices whose common total is

rational (equivalently, a perfect branching family inside one rational fiber).

The known disjoint (BL) trades provide the independence but have an irrational

lacunary total; the common-tail trick in the live comment provides a rational

total but only finitely many independent choices. Neither ingredient currently

supplies both properties at once.

PARTIAL: exact greedy constructions now verify every 1<=n<=100000, correct TUZ20's n=5588 table row, and reduce the uniform problem to a specific modular orbit; the all-n and rational-continuum questions remain open.

This is the AI working report, labelled by outcome — not an independently verified claim unless marked PROVED. ← ledger