A079772 Let C(n) be the n-th composite number; then a(n) is the smallest number > C(n) and not coprime to C(n).
6, 8, 10, 12, 12, 14, 16, 18, 18, 20, 22, 24, 24, 26, 30, 28, 30, 30, 32, 34, 36, 36, 40, 38, 40, 42, 42, 44, 46, 48, 48, 50, 56, 52, 54, 54, 56, 60, 58, 60, 60, 62, 64, 66, 66, 70, 68, 70, 72, 72, 74, 76, 78, 78, 84, 80, 82, 84, 84, 86, 90, 88, 90, 90, 92, 98, 94, 96, 96, 100
Offset: 1
Keywords
Examples
C(4) = 9 and C(5) = 10 hence a(4) = a(5) = 12. For 91=7*13 we have 91+7 = 98; for 92=2*2*23 we have 92-2 = 94
Programs
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PARI
cminusp31(n) = \sum c+min prime div { for(x=2,n, forprime(p=2,floor(sqrt(x)), if(x%p==0 & isprime(x)==0, print1(x+p,","); break); ) ) }
Formula
a(n) = C(n) + smallest prime divisor of C(n).
Extensions
More terms from Cino Hilliard, Aug 12 2003
Comments