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 A132952 a(n) is the number of isolated totatives of n. 4
 0, 1, 0, 2, 0, 2, 0, 4, 0, 4, 0, 4, 0, 6, 2, 8, 0, 6, 0, 8, 2, 10, 0, 8, 0, 12, 0, 12, 0, 8, 0, 16, 2, 16, 2, 12, 0, 18, 2, 16, 0, 12, 0, 20, 6, 22, 0, 16, 0, 20, 2, 24, 0, 18, 2, 24, 2, 28, 0, 16, 0, 30, 6, 32, 2, 20, 0, 32, 2, 24, 0, 24, 0, 36, 10, 36, 2, 24, 0, 32, 0, 40, 0, 24, 2, 42, 2, 40, 0 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,4 COMMENTS An isolated totative, k, of n is a positive integer which is less than and coprime to n and is such that neither (k-1) nor (k+1) are coprime to n. a(2n) = phi(2n), where phi(n) = A000010(n). If k is an isolated totative so is n-k. - Robert G. Wilson v, Sep 13 2007 a(n)=0 for n's: A061345 "Odd prime powers". - Robert G. Wilson v, Sep 13 2007 LINKS Antti Karttunen, Table of n, a(n) for n = 1..16384 Eric Weisstein's World of Mathematics, Totative. Antti Karttunen, Data supplement: n, a(n) computed for n = 1..65537 EXAMPLE The positive integers which are <= 15 and are coprime to 15 are 1,2,4,7,8,11,13,14. Of these, 1 and 2 are adjacent, 7 and 8 are adjacent and 13 and 14 are adjacent. So the isolated totatives of 15 are 4 and 11. There are 2 of these, so a(15) = 2. MATHEMATICA fQ[k_, n_] := GCD[k, n] == 1 && GCD[k - 1, n] > 1 && GCD[k + 1, n] > 1; f[n_] := Length@ Select[ Rest[ Range@n - 1], fQ[ #, n] &]; Array[f, 89] (* Robert G. Wilson v *) PROG (PARI) A132952(n) = { my(s=0, pg=0, g=1, ng); for(k=1, n-1, if((1!=(ng=gcd(n, k+1)))&&(1==g)&&(1!=pg), s++); pg = g; g = ng); (s); }; \\ Antti Karttunen, Nov 01 2018 CROSSREFS Cf. A132953. Sequence in context: A282849 A111813 A167156 * A319998 A336120 A029187 Adjacent sequences:  A132949 A132950 A132951 * A132953 A132954 A132955 KEYWORD nonn AUTHOR Leroy Quet, Sep 05 2007 EXTENSIONS Edited and extended by Robert G. Wilson v, Sep 13 2007 STATUS approved

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Last modified August 8 06:27 EDT 2020. Contains 336290 sequences. (Running on oeis4.)