A085273 -id:A085273 - cp's OEIS frontend

A085271 Difference between n-th composite number and its smallest prime divisor.

2, 4, 6, 6, 8, 10, 12, 12, 14, 16, 18, 18, 20, 22, 20, 24, 24, 26, 28, 30, 30, 32, 30, 34, 36, 36, 38, 40, 42, 42, 44, 46, 42, 48, 48, 50, 52, 50, 54, 54, 56, 58, 60, 60, 62, 60, 64, 66, 66, 68, 70, 72, 72, 74, 70, 76, 78, 78, 80, 82, 80, 84, 84, 86, 88, 84, 90, 90, 92, 90, 94

Offset: 1

Cino Hilliard, Aug 12 2003

			For composite number 91 we have 91-7 = 84.

Cf. A002808, A085273.

#-FactorInteger[#][[1,1]]&/@Complement[Range[2,200],Prime[Range[PrimePi[200]]]] (* Harvey P. Dale, Dec 11 2010 *)
#-Divisors[#][[2]]&/@Select[Range[4,200],!PrimeQ[#]&] (* Zak Seidov, Apr 09 2011 *)

cminusp(n) = { for(x=2,n, forprime(p=2,floor(sqrt(x)), if(x%p==0,print1(x-p,","); break); ) ) }

{n=200;for(x=4,n,if(isprime(x),,print1(x-divisors(x)[2], ", ")))} \\ Zak Seidov, Apr 09 2011

Typo in data corrected by Zak Seidov, Apr 09 2011

A085275 Sum of n-th composite number and its largest prime divisor.

Original entry on oeis.org

6, 9, 10, 12, 15, 15, 21, 20, 18, 21, 25, 28, 33, 27, 30, 39, 30, 35, 35, 34, 44, 51, 42, 39, 57, 52, 45, 49, 55, 50, 69, 51, 56, 55, 68, 65, 57, 66, 63, 76, 87, 65, 93, 70, 66, 78, 77, 85, 92, 77, 75, 111, 80, 95, 88, 91, 85, 84, 123, 91, 102, 129, 116, 99, 95, 104, 115, 124

Offset: 4

Cino Hilliard, Aug 12 2003

			For 91=7*13 we have 91+13 = 104; for 92=2*2*23 we have 92+23 = 115.

Cf. A085271, A085273.

cminusp3(n) = { for(x=2,n, forstep(p=x,2,-1, if(isprime(p) & x%p==0 & isprime(x)==0, print1(x+p,","); break); ) ) }

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A085271 Difference between n-th composite number and its smallest prime divisor.

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A085275 Sum of n-th composite number and its largest prime divisor.

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