TY - GEN

T1 - Fusible numbers and Peano Arithmetic

AU - Erickson, Jeff

AU - Nivasch, Gabriel

AU - Xu, Junyan

N1 - Publisher Copyright:
© 2021 IEEE.

PY - 2021/6/29

Y1 - 2021/6/29

N2 - Inspired by a mathematical riddle involving fuses, we define the fusible numbers as follows: 0 is fusible, and whenever x, y are fusible with |y - x| < 1, the number (x + y + 1)/2 is also fusible. We prove that the set of fusible numbers, ordered by the usual order on ℝ, is well-ordered, with order type ϵ0. Furthermore, we prove that the density of the fusible numbers along the real line grows at an incredibly fast rate: Letting g(n) be the largest gap between consecutive fusible numbers in the interval [n, ∞), we have g (n) - 1 \geq F_ \varepsilon _0(n - c) for some constant c, where Fα denotes the fast-growing hierarchy.Finally, we derive some true statements that can be formulated but not proven in Peano Arithmetic, of a different flavor than previously known such statements: PA cannot prove the true statement For every natural number n there exists a smallest fusible number larger than n. Also, consider the algorithm M(x): if x < 0 return -x, else return M(x - M(x - 1))/2. Then M terminates on real inputs, although PA cannot prove the statement M terminates on all natural inputs.

AB - Inspired by a mathematical riddle involving fuses, we define the fusible numbers as follows: 0 is fusible, and whenever x, y are fusible with |y - x| < 1, the number (x + y + 1)/2 is also fusible. We prove that the set of fusible numbers, ordered by the usual order on ℝ, is well-ordered, with order type ϵ0. Furthermore, we prove that the density of the fusible numbers along the real line grows at an incredibly fast rate: Letting g(n) be the largest gap between consecutive fusible numbers in the interval [n, ∞), we have g (n) - 1 \geq F_ \varepsilon _0(n - c) for some constant c, where Fα denotes the fast-growing hierarchy.Finally, we derive some true statements that can be formulated but not proven in Peano Arithmetic, of a different flavor than previously known such statements: PA cannot prove the true statement For every natural number n there exists a smallest fusible number larger than n. Also, consider the algorithm M(x): if x < 0 return -x, else return M(x - M(x - 1))/2. Then M terminates on real inputs, although PA cannot prove the statement M terminates on all natural inputs.

UR - http://www.scopus.com/inward/record.url?scp=85113905777&partnerID=8YFLogxK

U2 - 10.1109/LICS52264.2021.9470703

DO - 10.1109/LICS52264.2021.9470703

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AN - SCOPUS:85113905777

T3 - Proceedings - Symposium on Logic in Computer Science

BT - 2021 36th Annual ACM/IEEE Symposium on Logic in Computer Science, LICS 2021

PB - Institute of Electrical and Electronics Engineers Inc.

T2 - 36th Annual ACM/IEEE Symposium on Logic in Computer Science, LICS 2021

Y2 - 29 June 2021 through 2 July 2021

ER -