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
Offset: 1
Keywords
Examples
6*tau(6) = 24 hence 23 is in the sequence.
Links
- Amiram Eldar, Table of n, a(n) for n = 1..10000
Programs
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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 *) -
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