IAENG International Journal of Applied Mathematics, 46:2, IJAM_46_2_08 ______________________________________________________________________________________ A Solution to Yamakami’s Problem on Non-uniform Context-free Languages Toshio Suzuki, Member, IAENG Abstract—Yamakami (Theoret. Comput. Sci., 2011) studies is C-immune if no infinite subset of A belongs to C. Flajolet non-uniform context-free languages. Here, the length of advice and Steyaert observed that L3eq is CFL-immune. Being C- is assumed to be the same as that of an input. Let CFL and immune is a much stronger condition than non-membership CFL/n denote the class of all context-free languages and its non- uniform version, respectively. We let CFL(2) denote the class in C. Thus, the above observation shows that L3eq, a member of intersections of two context-free languages. An interesting of CFL(2), is far from belonging to CFL. direction of a research is asking how complex CFL(2) is, We consider another condition stronger than non- relative to CFL. Yamakami raised a problem whether there membership in a given class C. It is the non-membership is a CFL-immune set in CFL(2) - CFL/n. The best known so far is that LSPACE - CFL/n has a CFL-immune set, in a non-uniform version of C. In the usual mathematical where LSPACE denotes the class of languages recognized in model of computation, a fixed algorithm processes all the logarithmic-space. We present an affirmative solution to his inputs. Such a type of computation is called uniform. In the problem. Two key concepts of our proof are the overlapped case where the inputs are classified according to a certain palindrome and Yamakami’s swapping lemma. The swapping parameter, typically the input size, and a hardware is assigned lemma is applicable to the setting where the pumping lemma (Bar-Hillel’s lemma) does not work. Our proof is an example to each parameter, we need a stronger model. Such a type of showing how useful the swapping lemma is. In addition, by computation is called non-uniform. A typical case is given means of Kolmogorov complexity, we show the following: With by a family of circuits. Here, the family is not necessarily respect to realtime deterministic context-free languages, the computable. Non-uniform computation has meaningful ap- non-uniform class with parallel advice is not a subset of that plications. For example, in the studies of cryptography, non- with serial advice. uniform computation plays a role of a powerful adversary Index Terms—context-free language; pushdown automaton; [5]. advice function; non-uniform complexity class; immune set. A nice formulation of non-uniform computation is achieved by a computation with an advice function. An I. INTRODUCTION advice function is a function of a natural number to a string. HE regular languages have beautiful closure properties. An advice function is not necessarily computable. With an T For example, given two regular languages, their inter- input, the advice at the length of the input is provided to a section is a regular language. Nevertheless, in the studies fixed algorithm. Pippenger [13] characterizes computational of programming languages, most of important languages are power of polynomial-sized circuits by advice functions. Karp not regular. The same holds in the studies of formal models and Lipton [9] establish the foundation of computation with of natural languages. In the case of classes larger than the an advice function. Roughly speaking, given a class C of regular languages, closure properties are more difficult than languages and a size-bound β for advice functions, a non- the regular cases. uniform class C/β is defined. In most cases, even if all the In particular, given two context-free languages, their inter- members of C are computable languages, C/β contains non- section is not necessarily context free. For a positive integer computable languages. Thus, C/β is much larger than C. k, we consider the intersection of k context-free languages, Damm and Holzer [4] investigate, in the style of Karp and and let CFL(k) denote the class of all such intersections. Lipton, finite automata that take advice. In section IV, we CFL(1) is CFL, the class of all context-free languages. It is shall review their definitions. known that CFL(k) is a proper subset of CFL(k + 1). Tadaki et al. [15] investigate computation with advice in How complex is CFL(k + 1), relative to CFL(k)? An slightly different style from that of Karp and Lipton. Given interesting observation is given by Flajolet and Steyaert [6]. a class C of languages, we define C=n as follows. Suppose n n n Let L3eq denote the set of all strings of the form 0 1 2 , that L is a language over an alphabet Σ. Suppose that Γ is Σ where n is a natural number. It is easily seen that L3eq another alphabet. We introduce an extended alphabet . belongs to CFL(2). Γ x An immune set is a key concept in the classical recursion It consists of all symbols of the form for x 2 Σ theory, namely in Post’s problem. Later, immune sets relative a n to complexity classes are studied in the complexity theory and a 2 Γ. Given two strings x = x1 ··· xn 2 Σ and x [18]. Given a class C of languages, an infinite language A a = a ··· a 2 Γn of the same length, we let denote 1 n a n x x Σ Manuscript received September 13, 2015; revised January 14 and Febru- the string 1 ··· n 2 . ary 7, 2016. This work was partially supported by Japan Society for the a1 an Γ Promotion of Science (JSPS) KAKENHI (C) 22540146. Department of Mathematics and Information Sciences, Definition 1. (Tadaki et al. [15]) A language L belongs to Tokyo Metropolitan University, Minami-Ohsawa, Hachioji, 0 Tokyo 192-0397, Japan. e-mail: [email protected] (see C=n if and only if there exist a language L 2 C and a http://researchmap.jp/read0021048/?lang=english). function h : N ! Γ∗ such that for every x 2 Σ∗, the length (Advance online publication: 14 May 2016) IAENG International Journal of Applied Mathematics, 46:2, IJAM_46_2_08 ______________________________________________________________________________________ of h(jxj) is the same as that of x and the following holds. a real number x, dxe denotes the minimal natural number n ≥ x. x x 2 L , 2 L0 An alphabet denotes a finite set of characters. For an h(jxj) alphabet Σ, the set of all strings is denoted by Σ∗. The set + Then, h is called an advice function. h(n) is the advice at of all non-empty strings is denoted by Σ . Given a string length n. w, its length jwj denotes the total number of occurrences of characters. The reverse of w = w1 ··· wn, where n = jwj, is Here, the n of CFL/n denotes that the length of advice R wn ··· w1. The reverse of w is denoted by w . is same as the input length. The following are examples REG (CFL, respectively) is the class of all regular on DCFL, the class of languages accepted by deterministic (context-free) languages [8]. Suppose that C is a given class pushdown automata, and REG, the regular languages. In of languages such as REG or CFL. An advised class C=n is section IV, we will review more precise definition of DCFL. defined in the introduction. n n (i) Let Leq denote the set of all strings of the form 0 1 , When we discuss acceptance of an input by a pushdown where n is a natural number. Then Leq belongs to DCFL \ automaton, we consider acceptance by final state. During the REG=n and is REG-immune [6]. computation, the head has to scan all the letters of the input (ii) Let Pal# denote the palindromes whose center symbol string. At the end of computation, the state is a final state. is the separator symbol #. Then Pal# belongs to DCFL − We allow non-empty stack at the end of computation. REG=n and is REG-immune [17]. The class CFL/n is characterized by non-deterministic The CFL-immune set L3eq given in [6] belongs to CFL(2) pushdown automata with an advice function (Fig. 1). It has \ CFL=n; in order to verify that L3eq 2 CFL=n, consider a one-way read-only input tape and a pushdown memory n n n an advice function h(3n) = 0 1 2 . Thus, it is natural (stack). The input tape has two tracks. An input is given and interesting to ask whether there is a CFL-immune set on the first track. The advice at the length of the input is in CFL(2) − CFL=n. given on the second track. Then the automaton works as a Σ non-deterministic automaton over the alphabet . Yamakami’s problem Yamakami [17] raised a problem Γ whether there is a CFL-immune set in CFL(2) - CFL/n. In Fig. 1, zk ··· z1? is the string in the pushdown memory (stack). ? denotes the symbol denoting the bottom of the The best known so far is that LSPACE - CFL/n has a stack. ¢ is the left-end symbol. $ is the right-end symbol. CFL-immune set [17], where LSPACE denotes the class h(n) = a ··· a in the second track is the advice at length 1 n of languages recognized by deterministic Turing machines x1 n. The head is reading a symbol . q0 is the current with a single read-only input tape and a logarithmic-space a1 bounded work tape.