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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 Chai Wah Wu, Table of n, a(n) for n = 1..35 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 Adjacent sequences: A291298 A291299 A291300 * A291302 A291303 A291304 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.)