#!/usr/bin/env python3 # SPDX-License-Identifier: MIT # Copyright (c) 2026 Pablo Nogueira Grossi / G6 LLC """ strogatz-citations-verify.py -- check every Strogatz citation this corpus makes against the book itself. WHY. Several pages cite Strogatz by page and example number: book6/wp122, book7/ch-strogatz, and -- added 2026-09-17 -- ch-van-der-pol, ch-smale, ch-euler and ch-conley, written after this script existed. A page number is an address, and this corpus's standing rule is that an address is checked, not remembered. Until 2026-09-17 none of these had been opened against the source. The book is not in this repository and will not be. Supply your own copy: python3 book7/strogatz-citations-verify.py /path/to/strogatz.pdf EDITION. S. H. Strogatz, "Nonlinear Dynamics and Chaos", 2nd ed., Westview 2015 / CRC 2018, ISBN 978-0-8133-4910-7. Checked against the 2018 printing, 532 PDF pages, 481 printed pages located. METHOD. Printed page numbers are read off the top of each page rather than assumed from an offset -- the offset is NOT constant in this printing. The run reports it directly: three distinct values, 4 to 18. So any check that hard-codes one is checking the wrong page and will still pass on a lucky hit. Each claim then asserts that specific strings appear on that specific printed page. Page numbers are also not simply the leading integer: on recto pages the running head is set tight against the number, so "2818.7 POINCARE MAPS" is page 281 of section 8.7. Two pypdf versions disagree about the whitespace there, and a reader that did not handle it found only 278 of 513 pages under pypdf 6.18 and all of them under an older build -- a verifier whose verdict depends on its extractor is not a verifier. read_page_number() below is the fix. The located-page count depends on the extractor, so the run prints its pypdf version beside it. A count quoted without that version is not reproducible. Requires pypdf. Exit 0 iff every claim holds. """ import re, sys FRONT_MATTER_MAX = 40 # bound on (pdf index - printed page); it is 14-18 here EDITION = "Strogatz, Nonlinear Dynamics and Chaos, 2nd ed. (2018 printing)" # (citing file, printed page, [strings that must appear on it], note) CLAIMS = [ ("book7/ch-strogatz", 198, ["7.0", "limit cycle"], "section 7.0 opens Chapter 7"), ("book6/wp122 + ch-strogatz",199, ["Example 7.1.1", "limit cycle"], "Example 7.1.1, the r(1-r^2) limit cycle"), ("book6/wp122", 201, ["7.2"], "section 7.2, ruling out closed orbits"), ("book6/wp122", 203, ["7.2"], "still inside 7.2"), ("book6/wp122", 205, ["7.3"], "section 7.3, Poincare-Bendixson"), ("book6/wp122", 206, ["Example 7.3.1", "trapping region", "Bendixson"], "Example 7.3.1, the trapping annulus"), ("book7/ch-strogatz", 251, ["8.1", "8.2"], "Chapter 8 bifurcations revisited begins"), ("book7/ch-strogatz", 254, ["Rules of Thumb", "grows continuously from zero", "The frequency of the limit cycle"], "HEADING IS PLURAL: 'Rules of Thumb'. Rule 1 = amplitude, rule 2 = frequency"), ("book7/ch-strogatz", 256, ["degenerate Hopf bifurcation", "no limit cycles on either side", "continuous band of closed orbits", "nonlinear center"], "the degenerate case, damped pendulum"), ("book6/wp122", 281, ["8.7"], "section 8.7, Poincare maps"), ("book6/wp122", 282, ["EXAMPLE 8.7.1", "Exercise 8.7.1"], "Example 8.7.1 -- and the integral is deferred to EXERCISE 8.7.1"), ("book6/wp122", 373, ["10.5", "Liapunov"], "section 10.5, Liapunov exponent"), ("book6/wp122", 374, ["EXAMPLE 10.5.1", "stable p-cycle"], "Example 10.5.1, exponent of a p-cycle"), # ---- added 2026-09-17: the chapters written after this script existed. Their # ---- Strogatz page numbers were remembered, not checked, until this run. ("book7/ch-van-der-pol + ch-smale", 10, ["Figure 1.3.1", "RC circuit", "Radioactive decay", "Pendulum", "Strange attractors"], "Figure 1.3.1, the map of the subject -- and the cells both chapters quote"), ("book7/ch-van-der-pol", 200, ["EXAMPLE 7.1.2", "limit cycle"], "Example 7.1.2, the van der Pol oscillator, one page after 7.1.1"), ("book7/ch-van-der-pol", 212, ["7.4", "Liénard", "No Chaos in the Phase Plane"], "section 7.4 OPENS here with Lienard's equation -- but not the theorem"), ("book7/ch-van-der-pol", 213, ["EXAMPLE 7.4.1", "Liénard", "unique, stable limit cycle", "RELAXATION OSCILLATIONS"], "Lienard's THEOREM and its five conditions are on 213, not 212; 7.5 opens here too"), ("book7/ch-van-der-pol", 212, ["early days of nonlinear dynamics", "radio and vacuum tube", "mathematical life"], "the pull-quote the chapter prints -- a direct quotation is the strongest claim"), ("book7/ch-euler + ch-conley", 179, ["6.8", "INDEX THEORY", "6.8.1"], "section 6.8, index theory for closed curves"), ("book7/ch-euler", 180, ["Theorem 6.8.2", "enclose fixed points whose indices sum to"], "Theorem 6.8.2 is on 180 -- the statement ch-euler checks numerically"), ] def read_page_number(t): """Read the printed page number off the top of a page. Not as simple as a leading integer. On recto pages the running head is set tight against the number -- "2818.7 POINCARE MAPS" is page 281, section 8.7 -- so a naive \\d{1,3}\\b finds nothing there and a naive \\d+ finds 2818. Split the digit run so that what follows begins a section number. """ m = re.match(r"\s*(\d+)", t) if not m: return None digits, rest = m.group(1), t[m.end():] for k in (3, 2, 1): if len(digits) < k: continue n = int(digits[:k]) if not 1 <= n <= 600: continue if len(digits) == k: # number stands alone return n if re.match(r"^\d*\.\d", digits[k:] + rest): # remainder is "8.7", "10.5" return n return None def main(): if len(sys.argv) != 2: print(__doc__); return 2 try: import pypdf except ImportError: print("needs pypdf: pip install pypdf"); return 2 reader = pypdf.PdfReader(sys.argv[1]) printed, text = {}, {} for i, page in enumerate(reader.pages): try: t = page.extract_text() or "" except Exception: t = "" text[i] = re.sub(r"\s+", " ", t) n = read_page_number(text[i]) # A page of front matter, or a figure caption that happens to open with # digits, can yield a number that is not a page number. The printed page # can only lag its pdf index by the front matter, never lead it, and the # front matter here is tens of pages -- so bound the gap and the stray # reads drop out without a hand-kept exception list. if n is not None and 0 <= i - n <= FRONT_MATTER_MAX and n not in printed: printed[n] = i print(f" {EDITION}") print(f" {len(reader.pages)} pdf pages, {len(printed)} printed pages located " f"(pypdf {pypdf.__version__} -- the count depends on the extractor)") offs = sorted({i - n for n, i in printed.items()}) print(f" pdf-index minus printed-page takes {len(offs)} distinct values " f"({min(offs)}..{max(offs)}) -- which is why no offset is assumed\n") bad = 0 for who, page, needles, note in CLAIMS: idx = printed.get(page) if idx is None: print(f" FAIL p.{page:<4} printed page not found"); bad += 1; continue body = text[idx].lower() missing = [s for s in needles if s.lower() not in body] if missing: print(f" FAIL p.{page:<4} {who}") for s in missing: print(f" not on the page: {s!r}") bad += 1 else: print(f" ok p.{page:<4} {who:26s} {note}") print() if bad: print(f" {bad} of {len(CLAIMS)} citations do not check out."); return 1 print(f" all {len(CLAIMS)} citations check out against {EDITION}.") return 0 if __name__ == "__main__": sys.exit(main())