A108548 - OEIS

A108548

Fully multiplicative with a(prime(j)) = A108546(j), where A108546 is the lexicographically earliest permutation of primes such that after 2 the forms 4*k+1 and 4*k+3 alternate, and prime(j) is the j-th prime in A000040.

1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 13, 12, 11, 14, 15, 16, 17, 18, 19, 20, 21, 26, 29, 24, 25, 22, 27, 28, 23, 30, 37, 32, 39, 34, 35, 36, 31, 38, 33, 40, 41, 42, 43, 52, 45, 58, 53, 48, 49, 50, 51, 44, 47, 54, 65, 56, 57, 46, 61, 60, 59, 74, 63, 64, 55, 78, 73, 68, 87, 70, 67, 72

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OFFSET

1,2

COMMENTS

Multiplicative with a(2^e) = 2^e, else if p is the m-th prime then a(p^e) = q^e where q is the m/2-th prime of the form 4*k + 3 (A002145) for even m and a(p^e) = r^e where r is the (m-1)/2-th prime of the form 4*k + 1 (A002144) for odd m. - David A. Corneth, Apr 25 2022

Permutation of the natural numbers with fixed points A108549: a(A108549(n)) = A108549(n).

LINKS

Antti Karttunen, Table of n, a(n) for n = 1..26927

Index entries for sequences that are permutations of the natural numbers

MATHEMATICA

terms = 72;

A111745 = Module[{prs = Prime[Range[2 terms]], m3, m1, min},

m3 = Select[prs, Mod[#, 4] == 3&];

m1 = Select[prs, Mod[#, 4] == 1&];

min = Min[Length[m1], Length[m3]];

Riffle[Take[m3, min], Take[m1, min]]];

A108546[n_] := If[n == 1, 2, A111745[[n - 1]]];

A049084[n_] := PrimePi[n]*Boole[PrimeQ[n]];

a[n_] := If[n == 1, 1, Module[{p, e}, Product[{p, e} = pe; A108546[A049084[p]]^e, {pe, FactorInteger[n]}]]];

Array[a, terms] (* Jean-François Alcover, Nov 19 2021, using Harvey P. Dale's code for A111745 *)

PROG

(PARI)

up_to = 26927; \\ One of the prime fixed points.

A108546list(up_to) = { my(v=vector(up_to), p, q); v[1] = 2; v[2] = 3; v[3] = 5; for(n=4, up_to, p = v[n-2]; q = nextprime(1+p); while(q%4 != p%4, q=nextprime(1+q)); v[n] = q); (v); };

v108546 = A108546list(up_to);

A108546(n) = v108546[n];

A108548(n) = { my(f=factor(n)); f[, 1] = apply(A108546, apply(primepi, f[, 1])); factorback(f); }; \\ Antti Karttunen, Apr 25 2022

CROSSREFS

Cf. A002144, A002145, A049084, A108546, A108549 (fixed points), A332808 (inverse permutation).

Cf. also A332815, A332817 (this permutation applied to Doudna tree and its mirror image), also A332818, A332819.

Cf. also A267099, A332212 and A348746 for other similar mappings.

Sequence in context: A249157 A033621 A332808 * A333692 A333693 A322464

Adjacent sequences: A108545 A108546 A108547 * A108549 A108550 A108551

KEYWORD

nonn,mult,look

AUTHOR

Reinhard Zumkeller, Jun 10 2005

EXTENSIONS

Name edited by Antti Karttunen, Apr 25 2022

STATUS

approved