A079448 - OEIS

A079448

Primes p such that there is at least one integer x satisfying p = x*tau(x)-1 where tau(x) = A000005(x), the number of divisors of x.

3, 5, 11, 13, 23, 31, 37, 59, 61, 71, 73, 79, 83, 103, 107, 131, 139, 151, 157, 167, 191, 193, 227, 239, 263, 269, 277, 307, 311, 313, 347, 379, 397, 421, 431, 443, 449, 457, 479, 487, 491, 499, 541, 563, 571, 587, 593, 613, 619, 631, 643, 661, 673, 701, 719

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OFFSET

1,1

LINKS

Amiram Eldar, Table of n, a(n) for n = 1..10000

EXAMPLE

6*tau(6) = 24 hence 23 is in the sequence.

MATHEMATICA

seq[lim_] := Union[Select[Table[k * DivisorSigma[0, k] - 1, {k, 1, Floor[lim/2]+1}], # <= lim && PrimeQ[#] &]]; seq[1000] (* Amiram Eldar, Apr 19 2025 *)

PROG

(PARI) lista(nn) = {forprime(p=2, nn, for (n=1, p, if (n*numdiv(n)-1 == p, print1(p, ", "); break; ); ); ); } \\ Michel Marcus, Dec 01 2013

CROSSREFS

Cf. A000005, A079449.

Sequence in context: A078971 A266723 A129096 * A045407 A317309 A243897

Adjacent sequences: A079445 A079446 A079447 * A079449 A079450 A079451

KEYWORD

nonn

AUTHOR

Benoit Cloitre, Jan 13 2003

STATUS

approved