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 A141218 Write the n-th nonprime (A018252(n)) as a product of primes; decrease one copy of the largest prime by 1 and increase one copy of the smallest prime by 1, multiply the resulting numbers. 6
 1, 3, 6, 6, 8, 12, 12, 18, 16, 12, 18, 24, 24, 30, 24, 24, 36, 24, 36, 36, 24, 40, 48, 36, 36, 54, 48, 48, 54, 60, 48, 66, 48, 48, 60, 64, 72, 54, 60, 72, 72, 84, 72, 90, 72, 48, 72, 90, 96, 88, 90, 72, 108, 80, 108, 80, 108, 96, 72, 120, 108, 96, 126, 112, 120, 108, 96, 132, 120 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 LINKS Robert Israel, Table of n, a(n) for n = 1..10000 EXAMPLE 1st nonprime = 1 (has no prime factors); a(1) = empty product = 1. 2nd nonprime = 4 = (p(max)=2)*(p(min)=2); a(2) = (2-1)*(2+1) = 1*3 = 3. 3rd nonprime = 6 = (p(max)=3)*(p(min)=2); a(3) = (3-1)*(2+1) = 2*3 = 6. 4th nonprime = 8 = (p(max)=2)*(p=2)*(p(min)=2); a(4) = (2-1)*2*(2+1) = 1*2*3 = 6. MAPLE f:= proc(m) local F, p1, p2;   if isprime(m) then return NULL fi;   F:= numtheory:-factorset(m);   p1:= min(F); p2:= max(F);   m*(p1+1)/p1*(p2-1)/p2; end proc: 1, seq(f(i), i=2..200); # Robert Israel, Oct 08 2018 CROSSREFS Cf. A018252, A141553, A141554. Sequence in context: A178746 A229986 A025500 * A318845 A147866 A135702 Adjacent sequences:  A141215 A141216 A141217 * A141219 A141220 A141221 KEYWORD nonn,look AUTHOR Juri-Stepan Gerasimov, Aug 07 2008 EXTENSIONS Three terms corrected by R. J. Mathar, Aug 18 2008 Entry revised by N. J. A. Sloane, Mar 07 2014 Examples revised by Jon E. Schoenfield, Mar 08 2014 STATUS approved

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Last modified August 13 08:27 EDT 2020. Contains 336442 sequences. (Running on oeis4.)