An (n; s) Davenport-Schinzel sequence, for positive integers n and s, is a sequence composed of n distinct symbols with the properties that no two adjacent elements are equal, and that it does not contain, as a (possibly non-contiguous) subsequence, any alternation a \Delta \Delta \Delta b \Delta \Delta \Delta a \Delta \Delta \Delta b \Delta \Delta \Delta of length s + 2 between two distinct symbols a and b. The close relationship between Davenport-Schinzel sequences and the combinatorial structure of lower envelopes of collections of functions make the sequences very attractive because a variety of geometric problems can be formulated in terms of lower envelopes. A near-linear bound on the maximum length of Davenport-Schinzel sequences enable us to derive sharp bounds on the combinatorial structure underlying various geometric problems, which in turn yields efficient algorithms for these problems.

Recently some interesting first-order statements independent of Peano Arithmetic (P) have been found. Here we present perhaps the first which is, in an informal sense, purely number-theoretic in character (as opposed to metamathematical or combinatorial). The methods used to prove it, however, are combinatorial. We also give another independence result (unashamedly combinatorial in character) proved by the same methods. The first result is an improvement of a theorem of Goodstein [2]. Let m and n be natural numbers, n> 1. We define the base n representation of m as follows: First write m as the sum of powers of n. (For example, if m = 266, n = 2, write 266 = 2 8 + 2 3 + 2 1.) Now write each exponent as the sum of powers of n. (For example, 266 = 2 23 + 2 2 + 1 +2 1.) Repeat with exponents of exponents and so on until the representation stabilizes. For example, 266 stabilizes at the representation 2 * +l + 2 2 + l +2 l. We now define the number Gn(m) as follows. If m = 0 set Gn(m) = 0. Otherwise set Gn(m) to be the number produced by replacing every n in the base n representation of m by n +1 and then subtracting 1. (For example, G2(266) = 3 33+1 + 3 3 + 1 +2). Now define the Goodstein sequence for m starting at 2 by So, for example, m0 = m, mx = G2{m0), m2 = G^mJ, m3 = G^m2),.... 266O = 266 = 2 22+1 + 2 2+1 + 2 X = 3 33+1 + 3 3+1 + 2 ~ 1O 38 2662 = 4 44+1 + 4 4+1 + l ~ 10 616 2663 = 5 s5+1 + 5 5+1 ~ 10 10- 000. Similarly we can define the Goodstein sequence for m starting at n for any n> 1. THEOREM 1. (i) (Goodstein [2]) Vm 3/c mk = 0. More generally for any m, n> 1 the Goodstein sequence for m starting at n eventually hits zero. (ii) Vm 3k mk = 0 (formalized in the language of first order arithmetic) is not provable

Let f be a number-theoretic function. A finite set X of natural numbers is called f-large if card(X) ≥ f(min(X)). Let PHf be the Paris Harrington statement where we replace the largeness condition by a corresponding f-largeness condition. We classify those functions f for which the statement PHf is independent of first order (Peano) arithmetic PA.Iffis a fixed iteration of the binary length function, then PHf is independent. On the other hand PHlog ∗ is provable in PA. More precisely let fα(i):=|i | H −1 α (i) where | i |h denotes the h-times iterated binary length of i and H−1 α denotes the inverse function of the α-th member Hα of the Hardy hierarchy. Then PHfα is independent of PA (for α ≤ ε0) iffα = ε0.

. We describe a model-theoretic approach to ordinal analysis via the finite combinatorial notion of an #-large set of natural numbers. In contrast to syntactic approaches that use cut elimination, this approach involves constructing finite sets of numbers with combinatorial properties that, in nonstandard instances, give rise to models of the theory being analyzed. This method is applied to obtain ordinal analyses of a number of interesting subsystems of first- and second-order arithmetic. x1. Introduction. Two of proof theory's defining goals are the justification of classical theories on constructive grounds, and the extraction of constructive information from classical proofs. Since Gentzen, ordinal analysis has been a major component in these pursuits, and the assignment of recursive ordinals to theories has proven to be an illuminating way of measuring their constructive strength. The traditional approach to ordinal analysis, which uses cut-elimination procedures to transfor...

We use the Boyer-Moore Prover, Nqthm, to verify the Paris-Harrington version of Ramsey's Theorem. The proof we verify is a modification of the one given by Ketonen and Solovay. The Theorem is not provable in Peano Arithmetic, and one key step in the proof requires ffl 0 induction. x0. Introduction. The most well-known formalizations of finite mathematics are PA (Peano Arithmetic) and PRA (Primitive Recursive Arithmetic). In both, the "intended" domain of discourse is the set of natural numbers. PA is formalized in standard first-order logic, and contains the induction schema, which can apply to arbitrary first-order formulas. The logic of PRA allows only quantifier-free formulas, which are thought of as being universally quantified, and PRA has the induction scheme for quantifier-free formulas, expressed as a proof rule. Also, for each primitive recursive function f , PRA contains a function symbol for f and has the recursive definition of f as an axiom. Clearly, PRA is much weaker tha...

We provide a set of "natural" requirements for well-orderings of (binary) list structures. We show that the resultant order-type is the successor of the first critical epsilon number. The checker has to verify that the process comes to an end. Here again he should be assisted by the programmer giving a further definite assertion to be verified. This may take the form of a quantity which is asserted to decrease continually and vanish when the machine stops. To the pure mathematician it is natural to give an ordinal number. In this problem the ordinal might be (n \Gamma r)! 2 + (r \Gamma s)! + k. A less highbrow form of the same thing would be to give the integer 2 80 (n \Gamma r) + 2 40 (r \Gamma s) + k. ---Alan M. Turing (1949) 1 Introduction A riddle---consider the Lisp-like function f , f(a) = a f(b) = b Research supported in part by the National Science Foundation under Grants CCR90 -07195 and CCR-90-24271. f(cons(x; y)) = 8 ? ? ? ? ? ? ? ? ? ? ! ? ? ? ? ? ? ? ? ? ?...

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This paper presents a new proof of the characterization of the provably recursive functions of the fragments I# n of Peano arithmetic. The proof method also characterizes the # k -definable functions of I# n and of theories axiomatized by transfinite induction on ordinals. The proofs are completely proof-theoretic and use the method of witness functions and witness oracles.