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A286095 Composite numbers n such that tau(n) (number of divisors of n) is prime and sigma(n) (sum of divisors of n) is not prime. 1
49, 81, 121, 169, 361, 529, 625, 841, 961, 1024, 1369, 1849, 2209, 2809, 3721, 4489, 5329, 6241, 6889, 9409, 10609, 11449, 11881, 12769, 14641, 16129, 18769, 19321, 22201, 22801, 24649, 26569, 32041, 32761, 36481, 37249, 38809, 39601, 44521, 49729, 51529, 52441, 54289 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

If sigma(n) is prime (A023194) then tau(n) is prime too. (See Crux Mathematicorum link.)

But the reverse is false; the numbers which verify tau(n) prime and sigma(n) not prime are in the sequence A275938.

All odd primes belong to the sequence A275938, but there are also in this sequence composite numbers which are all prime powers, these prime powers are here.

LINKS

Robert Israel, Table of n, a(n) for n = 1..10000

Peter A. Lindstrom and Andy Liu, Problem 465, Crux Mathematicorum, page 188, Vol.6 , Jun. 80.

EXAMPLE

tau(49) = 3 and sigma(49) = 57 = 3 * 19.

MAPLE

for n from 2 to 550000 do p(n):=tau(n);

if not isprime(n) and is prime(p(n)) and not isprime(sigma(n)) then print (n, p(n), sigma(n)) else fi; od:

# alternative

N:= 10^5: # to get all terms <= N

P:= select(isprime, [2, seq(i, i=3..isqrt(N), 2)]):

S:= {}:

for p in P do

  k:= 1:

  do

    k:= nextprime(k+1)-1;

    if p^k > N then break fi;

    if not isprime((p^(k+1)-1)/(p-1)) then S:= S union {p^k} fi

  od

od:

sort(convert(S, list)); # Robert Israel, Jun 05 2017

MATHEMATICA

Select[Range[10^5], Function[n, And[CompositeQ@ n, Map[PrimeQ@ DivisorSigma[#, n] &, {0, 1}] == {True, False}]]] (* Michael De Vlieger, May 24 2017 *)

PROG

(PARI) lista(nn) = {forcomposite(n=1, nn, if (isprime(numdiv(n)) && !isprime(sigma(n)), print1(n, ", ")); ); } \\ Michel Marcus, May 24 2017

CROSSREFS

Cf. A000005, A000203, A023194, A275938.

Sequence in context: A056938 A267986 A207638 * A106311 A006832 A250074

Adjacent sequences:  A286092 A286093 A286094 * A286096 A286097 A286098

KEYWORD

nonn

AUTHOR

Bernard Schott, May 22 2017

STATUS

approved

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Last modified February 17 16:03 EST 2018. Contains 299296 sequences. (Running on oeis4.)