login
The OEIS is supported by the many generous donors to the OEIS Foundation.

 

Logo
Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A079707 In prime factorization of n replace odd primes with their prime predecessor. 1

%I #11 Nov 29 2022 02:43:07

%S 1,2,2,4,3,4,5,8,4,6,7,8,11,10,6,16,13,8,17,12,10,14,19,16,9,22,8,20,

%T 23,12,29,32,14,26,15,16,31,34,22,24,37,20,41,28,12,38,43,32,25,18,26,

%U 44,47,16,21,40,34,46,53,24,59,58,20,64,33,28,61,52,38,30,67,32,71,62,18

%N In prime factorization of n replace odd primes with their prime predecessor.

%C Result after A061395(n)-1 iterations = A061142(n).

%H Amiram Eldar, <a href="/A079707/b079707.txt">Table of n, a(n) for n = 1..10000</a>

%F a(n) <= n; a(n) < n iff n > 1 is odd; a(n) = n iff n = 2^k.

%F A001222(a(n)) = A001222(n).

%F For 3-smooth numbers: a(2^i * 3^j) = 2^(i+j).

%F Multiplicative with 2->2 and prime(k)->prime(k-1), k>1.

%F Sum_{k=1..n} a(k) ~ c * n^2, where c = (1/2) * Product_{p prime > 2} ((p^2-p)/(p^2 - prevprime(p))) = 0.3310558934..., where prevprime is A151799. - _Amiram Eldar_, Nov 29 2022

%t f[p_, e_] := If[p == 2, 2, NextPrime[p, -1]]^e; a[1] = 1; a[n_] := Times @@ f @@@ FactorInteger[n]; Array[a, 100] (* _Amiram Eldar_, Nov 29 2022 *)

%o (PARI) a(n) = {my(f = factor(n)); prod(i = 1, #f~, if(f[i,1] == 2, f[i,1], precprime(f[i,1]-1))^f[i,2]);} \\ _Amiram Eldar_, Nov 29 2022

%Y Cf. A001222, A005408, A000079, A003586, A003961, A061142, A061395, A064989, A151799.

%K nonn,easy,mult

%O 1,2

%A _Reinhard Zumkeller_, Jan 31 2003

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified March 28 05:39 EDT 2024. Contains 371235 sequences. (Running on oeis4.)