|
| |
|
|
A030664
|
|
Product of largest prime <= n and smallest prime >= n.
|
|
5
| |
|
|
1, 1, 4, 9, 15, 25, 35, 49, 77, 77, 77, 121, 143, 169, 221, 221, 221, 289, 323, 361, 437, 437, 437, 529, 667, 667, 667, 667, 667, 841, 899, 961, 1147, 1147, 1147, 1147, 1147, 1369, 1517, 1517, 1517, 1681, 1763, 1849, 2021, 2021, 2021, 2209, 2491, 2491, 2491
(list; graph; refs; listen; history; internal format)
|
|
|
|
OFFSET
| 0,3
|
|
|
COMMENTS
| Symmetrical about zero, a(n)=a(-n) if n>1, if you recognize negative primes. - Robert G. Wilson v, Feb 28 2011
Iff n is a prime then a(n)=n^2, otherwise a(n) is a semiprime. - Robert G. Wilson v, Feb 28 2011
|
|
|
LINKS
| T. D. Noe, Table of n, a(n) for n=0..1000
|
|
|
MATHEMATICA
| f[n_] := If[Abs[n] < 2, 1, NextPrime[n + 1, -1] NextPrime[n - 1]]; Array[f, 51, 0] (* Robert G. Wilson v, Feb 28 2011 *)
|
|
|
PROG
| (Mupad) numlib::prevprime(i)*nextprime(i) $ i = 0..50 - Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Feb 26 2007
(Haskell)
a030664 n = a030664_list !! n
a030664_list = f 1 (1:a000040_list) [0..] where
f q (p:ps) (n:ns) | n < p = q*p : f q (p:ps) ns
| otherwise = p^2 : f p ps ns
-- Reinhard Zumkeller, Feb 24 2011
|
|
|
CROSSREFS
| Cf. A000040, A013638, A006094, A001248.
Sequence in context: A050530 A102084 A193315 * A070160 A056928 A122964
Adjacent sequences: A030661 A030662 A030663 * A030665 A030666 A030667
|
|
|
KEYWORD
| easy,nonn,nice
|
|
|
AUTHOR
| N. J. A. Sloane (njas(AT)research.att.com).
|
|
|
EXTENSIONS
| More terms from Erich Friedman (erich.friedman(AT)stetson.edu).
|
| |
|
|
