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A134344 Composite numbers such that the arithmetic mean of their prime factors (counted with multiplicity) is prime. 28
4, 8, 9, 16, 20, 21, 25, 27, 32, 33, 44, 49, 57, 60, 64, 68, 69, 81, 85, 93, 105, 112, 116, 121, 125, 128, 129, 133, 145, 156, 169, 177, 180, 188, 195, 205, 212, 213, 217, 220, 231, 237, 243, 249, 253, 256, 265, 272, 275, 289, 297, 309, 332, 336, 343, 356, 361 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Originally, the definition started with "Nonprime numbers ...". This may be misleading, since 1 is also nonprime, but has no prime factors. - Hieronymus Fischer, May 05 2013

LINKS

Harvey P. Dale and Hieronymus Fischer, Table of n, a(n) for n = 1..10000 (first 1000 terms from Harvey P. Dale)

EXAMPLE

a(1) = 4, since 4 = 2*2 and the arithmetic mean (2+2)/2 = 2 is prime.

a(5) = 20, since 20 = 2*2*5 and the arithmetic mean (2+2+5)/3 = 3 is prime.

MATHEMATICA

ampfQ[n_]:=PrimeQ[Mean[Flatten[Table[#[[1]], {#[[2]]}]&/@FactorInteger[ n]]]]; nn=400; Select[Complement[Range[nn], Prime[Range[ PrimePi[nn]]]], ampfQ] (* Harvey P. Dale, Nov 06 2012 *)

PROG

(PARI) is(n)=if(n<4, return(0)); my(f=factor(n), s=sum(i=1, #f~, f[i, 1]*f[i, 2])/sum(i=1, #f~, f[i, 2])); (#f~>1 || f[1, 2]>1) && denominator(s)==1 && isprime(s) \\ Charles R Greathouse IV, Sep 14 2015

CROSSREFS

Cf. A000040, A001222, A100118, A046363, A133620, A133621, A133880, A133890, A133900, A133910, A133911, A134330, A134331, A134332, A134333, A134334.

Sequence in context: A003679 A079432 A162215 * A119315 A010390 A003624

Adjacent sequences:  A134341 A134342 A134343 * A134345 A134346 A134347

KEYWORD

nonn

AUTHOR

Hieronymus Fischer, Oct 23 2007

EXTENSIONS

Definition clarified by Hieronymus Fischer, May 05 2013

STATUS

approved

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Last modified March 27 16:36 EDT 2017. Contains 284177 sequences.