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 A215659 Values of k such that k*(k - 1) is a primorial number. 5
 2, 3, 6, 15, 715 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Values of k in A215658. See A161620 for the primorial values. - Jeppe Stig Nielsen, Mar 27 2018 LINKS Carol Nelson, David E. Penney and Carl Pomerance, 714 and 715, J. Recreational Math. 7:2 (1994), pp. 87-89. FORMULA a(n) * (a(n) - 1) = A215658(n)#, where p# = 2 * 3 * 5 * 7 * 11 * ... * p is a primorial, the product of the primes from 2 to p. MATHEMATICA Select[Range[10^5], Product[Prime@ i, {i, PrimeNu@ #}] == # &[# (# - 1)] &] (* Michael De Vlieger, Apr 10 2018 *) CROSSREFS Cf. A215658, A161620. Sequence in context: A014627 A145781 A109162 * A028688 A030753 A003040 Adjacent sequences:  A215656 A215657 A215658 * A215660 A215661 A215662 KEYWORD nonn,more AUTHOR Jonathan Sondow, Sep 07 2012 EXTENSIONS Name improved by Jeppe Stig Nielsen, Mar 27 2018 STATUS approved

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Last modified March 30 19:49 EDT 2020. Contains 333127 sequences. (Running on oeis4.)