The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A036689 Product of a prime and the previous number. 43
 2, 6, 20, 42, 110, 156, 272, 342, 506, 812, 930, 1332, 1640, 1806, 2162, 2756, 3422, 3660, 4422, 4970, 5256, 6162, 6806, 7832, 9312, 10100, 10506, 11342, 11772, 12656, 16002, 17030, 18632, 19182, 22052, 22650, 24492, 26406, 27722, 29756, 31862, 32580, 36290, 37056, 38612, 39402, 44310 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Records in A002618. - Artur Jasinski, Jan 23 2008 Also records in A174857. - Vladimir Shevelev, Mar 31 2010 LINKS Reinhard Zumkeller, Table of n, a(n) for n = 1..10000 FORMULA a(n) = prime(n) * (prime(n) - 1). a(n) = phi(prime(n)^2) = A000010(A001248(n)). a(n) = prime(n) * phi(prime(n)). - Artur Jasinski, Jan 23 2008 From Reinhard Zumkeller, Sep 17 2011: (Start) a(n) = A000040(n) * A006093(n) = A001248(n) - A000040(n). A006530(a(n)) = A000040(n). (End) a(n) = A009262(prime(n)). - Enrique Pérez Herrero, May 12 2012 a(n) = prime(n)! mod (prime(n)^2). - J. M. Bergot, Apr 10 2014 a(n) = 2* A008837(n). - Antti Karttunen, May 01 2015 Sum_{n>=1} 1/a(n) = A136141. - Amiram Eldar, Nov 09 2020 From Amiram Eldar, Jan 23 2021: (Start) Product_{n>=1} (1 + 1/a(n)) = zeta(2)*zeta(3)/zeta(6) (A082695). Product_{n>=1} (1 - 1/a(n)) = A005596. (End) EXAMPLE 2*1, 3*2, 5*4, 7*6, 11*10, 13*12, 17*16, ... MAPLE A036689 := proc(n) local p ; p := ithprime(n) ; p*(p-1) ; end proc: # R. J. Mathar, Apr 11 2011 MATHEMATICA Table[Prime[n] EulerPhi[Prime[n]], {n, 100}] (* Artur Jasinski, Jan 23 2008 *) Table[Prime[n] (Prime[n] - 1), {n, 1, 50}] (* Bruno Berselli, Apr 22 2014 *) #(#-1)&/@Prime[Range[50]] (* Harvey P. Dale, Sep 08 2019 *) PROG (MAGMA) [ n*(n-1): n in PrimesUpTo(220) ]; // Bruno Berselli, Apr 11 2011 (PARI) forprime(p=2, 1e3, print1(p^2-p", ")) \\ Charles R Greathouse IV, Jun 10 2011 (Haskell) a036689 n = a036689_list !! (n-1) a036689_list = zipWith (*) a000040_list \$ map pred a000040_list -- Reinhard Zumkeller, Sep 17 2011 (Scheme) (define (A036689 n) (* (A000040 n) (- (A000040 n) 1))) ;; Antti Karttunen, May 01 2015 CROSSREFS Cf. A000040, A001248, A002618, A005596, A053650, A053192, A053193, A053650, A082695, A117495, A136141. Twice the terms of A008837. Subsequence of A002378 (oblong numbers). Column 1 of A257251. (Row 1 of A257252.) Sequence in context: A323724 A214307 A087134 * A226326 A139115 A193538 Adjacent sequences:  A036686 A036687 A036688 * A036690 A036691 A036692 KEYWORD nonn,easy AUTHOR EXTENSIONS Deleted two incorrect comments. - N. J. A. Sloane, May 07 2020 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified April 19 00:03 EDT 2021. Contains 343098 sequences. (Running on oeis4.)