The On-Line Encyclopedia of Integer Sequences (OEIS)

A246277 revision #70

A246277

Column index of n in A246278: a(1) = 0, a(2n) = n, a(2n+1) = a(A064989(2n+1)).

0, 1, 1, 2, 1, 3, 1, 4, 2, 5, 1, 6, 1, 7, 3, 8, 1, 9, 1, 10, 5, 11, 1, 12, 2, 13, 4, 14, 1, 15, 1, 16, 7, 17, 3, 18, 1, 19, 11, 20, 1, 21, 1, 22, 6, 23, 1, 24, 2, 25, 13, 26, 1, 27, 5, 28, 17, 29, 1, 30, 1, 31, 10, 32, 7, 33, 1, 34, 19, 35, 1, 36, 1, 37, 9, 38, 3, 39, 1, 40, 8, 41, 1, 42

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OFFSET

1,4

COMMENTS

If n >= 2, n occurs in column a(n) of A246278.

By convention, a(1) = 0 because 1 does not occur in A246278.

LINKS

Antti Karttunen, Table of n, a(n) for n = 1..10000

Index entries for sequences computed from indices in prime factorization

FORMULA

a(1) = 0, a(2n) = n, a(2n+1) = a(A064989(2n+1)) = a(A064216(n+1)). [Cf. the formula for A252463.]

Instead of the equation for a(2n+1) above, we may write a(A003961(n)) = a(n). - Peter Munn, May 21 2022

Other identities. For all n >= 1, the following holds:

For all w >= 0, a(p_{i} * p_{j} * ... * p_{k}) = a(p_{i+w} * p_{j+w} * ... * p_{k+w}).

For all n >= 2, A001222(a(n)) = A001222(n)-1. [a(n) has one less prime factor than n. Thus each semiprime (A001358) is mapped to some prime (A000040), etc.]

For all n >= 2, a(n) = A078898(A249817(n)).

For semiprimes n = p_i * p_j, j >= i, a(n) = A000040(1+A243055(n)) = p_{1+j-i}.

a(n) = floor(A348717(n)/2). - Antti Karttunen, Apr 30 2022

If n has prime factorization Product_{i=1..k} prime(x_i), then a(n) = Product_{i=2..k} prime(x_k-x_1+1). The opposite version is A358195, prime indices A358172, even bisection A241916. - Gus Wiseman, Dec 29 2022

MATHEMATICA

a246277[n_Integer] := Module[{f, p, a064989, a},

f[x_] := Transpose@FactorInteger[x];

p[x_] := Which[

x == 1, 1,

x == 2, 1,

True, NextPrime[x, -1]];

a064989[x_] := Times @@ Power[p /@ First[f[x]], Last[f[x]]];

a[1] = 0;

a[x_] := If[EvenQ[x], x/2, NestWhile[a064989, x, OddQ]/2];

a/@Range[n]]; a246277[84] (* Michael De Vlieger, Dec 19 2014 *)

PROG

(PARI)

A064989(n) = {my(f); f = factor(n); if((n>1 && f[1, 1]==2), f[1, 2] = 0); for (i=1, #f~, f[i, 1] = precprime(f[i, 1]-1)); factorback(f)};

A246277(n) = { if(1==n, 0, while((n%2), n = A064989(n)); (n/2)); };

(PARI) A246277(n) = if(1==n, 0, my(f = factor(n), k = primepi(f[1, 1])-1); for (i=1, #f~, f[i, 1] = prime(primepi(f[i, 1])-k)); factorback(f)/2); \\ Antti Karttunen, Apr 30 2022

(Scheme) ;; two different variants, the second one employing memoizing definec-macro)

(define (A246277 n) (if (= 1 n) 0 (let loop ((n n)) (if (even? n) (/ n 2) (loop (A064989 n))))))

(definec (A246277 n) (cond ((= 1 n) 0) ((even? n) (/ n 2)) (else (A246277 (A064989 n)))))

(Python)

from sympy import factorint, prevprime

from operator import mul

from functools import reduce

def a064989(n):

f=factorint(n)

return 1 if n==1 else reduce(mul, [1 if i==2 else prevprime(i)**f[i] for i in f])

def a(n): return 0 if n==1 else n//2 if n%2==0 else a(a064989(n))

print([a(n) for n in range(1, 101)]) # Indranil Ghosh, Jun 15 2017

CROSSREFS

Terms of A348717 halved. A305897 is the restricted growth sequence transform.

Positions of terms 1 .. 8 in this sequence are given by the following sequences: A000040, A001248, A006094, A030078, A090076, A251720, A090090, A030514.

Cf. A078898 (has the same role with array A083221 as this sequence has with A246278).

Cf. A000040, A001222, A001358, A003961, A055396, A064989, A064216, A243055, A246272, A249810, A249820, A249735, A252463.

This sequence is also used in the definition of the following permutations: A246274, A246276, A246675, A246677, A246683, A249815, A249817 (A249818), A249823, A249825, A250244, A250245, A250247, A250249.

Also in the definition of arrays A249821, A251721, A251722.

The sum of prime indices of a(n) is A359358 + A001222 - 1, cf. A326844.

A112798 lists prime indices, length A001222, sum A056239.

Cf. A005940, A019565, A161511, A241916, A243503, A246277, A253565, A356958, A358137, A358195.

Sequence in context: A325352 A305438 A078898 * A260739 A130747 A370822

Adjacent sequences: A246274 A246275 A246276 * A246278 A246279 A246280

KEYWORD

nonn

AUTHOR

Antti Karttunen, Aug 21 2014

STATUS

editing