Let X, Y be classes of primitive recursive functions (for short PRF) and let it be required to prove the following statement: (I) It is not true that every function of class X belongs to class Y. Such an assertion is usually proved by exhibiting a PRF of class X which grows faster than any PRF of class Y, or by constructing some “simple” PRF's, e.g., λx(x + 1); λxy(x + y), from class X, which do not belong to class Y. A method is proposed which gives a proof of an assertion of type (I) for some classes of PRF's X and Y such that first, functions of class X do not increase faster than functions of class Y, and second, the class Y contains “simple” PRF's such as λxy(x + y), λxy(x−y), etc. The proposed method is as follows. We choose some class of PRF's Z and for each PRF f we construct an operation δf on the functions of the class Z such that for every f in Y, the class Z is closed with respect to the operation δf, but on the other hand it is not true that for every f of X the class Z is closed with respect to δf. We describe one of the possible applications of the method. We shall not distinguish between word and number PRF's and predicates, bearing in mind the following one-to-one correspondence between words in the alphabet {1, 2} and natural numbers: the word anan−1... a1a0 in the alphabet {1, 2} corresponds to the number $$\sum\limits_{i = 0}^n {a_i 2^i } $$ . Let ν (x, i) equal the i-th letter in the word x (the last letter-is taken to be the 0-th sign), ¦x¦ is the length of x; con(x, y) is the result of concatenating the word y to the right of the word x; θ, ¯θ are the characteristic functions of equality and inequality. Let RF be a class of PRF's obtained from the PRF's θ, ¯θ, con, λx0, σ, I k n , by the operation of bounded minimalization and composition of functions. We note that the class of relations of the class RF is equal to the class of rudimentary relations. Let RsF(f1, ..., fl) be a class of PRF's obtained from the PRF's f1, ..., fl by the operation of composition and the operation of putting into correspondence with the function g an f such that $$f(\bar x,y) = \mu t_{ \leqslant y} [tPy\& g(\bar x,t) = 0]$$ , where tPy ⇋ t is the subword y. We note that the characteristic function of every s-rudimentary predicate belongs to RsF(θ, θ, con, λx0, σ, I k n ). We take as Z the class of such α in RF which have the values 0, 1, 2 and if $$\alpha (\bar z,i) = 0$$ then $$\forall k[\alpha (\bar z,i + k) = 0]$$ . The operation δf is such that $$[\delta _f (\alpha _1 ,...,\alpha _n )](\bar x_1 \bar z_1 j) = v(f(\sum\limits_{i = 0}^{x_1 } {\alpha _1 (\bar z,i)2^i ,...,\sum\limits_{i = 0}^{x_n } {\alpha _n (\bar z,i)2^i ),j} )} $$ . Assertions of type (I) can be proved by the proposed method if for X we take RF, and as Y we take $$Y - R_s F(\theta ,\bar \theta ,con,\lambda x0\sigma ,I_k^n ,\lambda xy(x + y),\lambda xy(x\dot - y),\lambda x\lambda f(|x|),\lambda x2^{g(|x|)} )$$ , where f and g belong to the class RF.
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