A070801 Largest prime <= sigma(n): a(n) = prevprime(sigma(n)), where prevprime(n) = A007917(n), the largest prime less than or equal to n.
3, 3, 7, 5, 11, 7, 13, 13, 17, 11, 23, 13, 23, 23, 31, 17, 37, 19, 41, 31, 31, 23, 59, 31, 41, 37, 53, 29, 71, 31, 61, 47, 53, 47, 89, 37, 59, 53, 89, 41, 89, 43, 83, 73, 71, 47, 113, 53, 89, 71, 97, 53, 113, 71, 113, 79, 89, 59, 167, 61, 89, 103, 127, 83, 139, 67, 113, 89
Offset: 2
Examples
For n=100: sigma(100) = 217, prevprime(217) = 211 = a(100).
Links
- Antti Karttunen, Table of n, a(n) for n = 2..16385
Crossrefs
Programs
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Mathematica
Table[Prime[PrimePi[DivisorSigma[1, w]]], {w, 2, 128}] Table[NextPrime[DivisorSigma[1, n] + 1, -1], {n, 2, 128}] (* Amiram Eldar, Mar 01 2024 *) -
PARI
A070801(n) = precprime(sigma(n)); \\ Antti Karttunen, Nov 07 2017
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Scheme
(define (A070801 n) (let ((s1 (+ 1 (A000203 n)))) (let loop ((k s1)) (if (<= (A000203 k) s1) k (loop (- k 1)))))) ;; (For code of A000203, see under that entry). Antti Karttunen, Nov 07 2017
Comments