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A291301 a(n) = prime that is eventually reached when x -> sigma(x)-1 is repeatedly applied to the product of the first n primes, or -1 if no prime is ever reached. 5
2, 11, 71, 743, 6911, 117239, 2013983, 34836479, 921086711, 33596203871, 18754852859999, 1306753691335679, 2795529813471359, 200489563747397471, 7143750592470475271, 146095655504943513599, 161739770170976834876927, 543475838478389870591999, 317180662337566737324195839 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
A subsequence of A039654.
LINKS
EXAMPLE
2*3*5*7*11*13 = 30030 -> 96767 -> 111359 -> 117239 takes three steps to reach a prime, so a(6) = 117239.
MATHEMATICA
p[n_]:=Times@@Prime/@Range[n]; f[n_]:=DivisorSigma[1, n]-1;
a[n_]:=Last[NestWhileList[f, p[n], CompositeQ]]; a/@Range[20] (* Ivan N. Ianakiev, Sep 01 2017 *)
PROG
(Python)
from sympy import primorial, isprime, divisor_sigma
def A291301(n):
m = primorial(n)
while not isprime(m):
m = divisor_sigma(m) - 1
return m # Chai Wah Wu, Aug 31 2017
CROSSREFS
Cf. A039654, A000203, A002110, A291302 (number of steps).
Sequence in context: A047776 A214692 A186633 * A154887 A371518 A363389
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Aug 31 2017
EXTENSIONS
a(10)-a(19) from Chai Wah Wu, Aug 31 2017
STATUS
approved

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Last modified April 16 04:17 EDT 2024. Contains 371696 sequences. (Running on oeis4.)