A085273 Difference between n-th composite number and its largest prime divisor.

2, 3, 6, 6, 5, 9, 7, 10, 14, 15, 15, 14, 11, 21, 20, 13, 24, 21, 25, 30, 22, 17, 28, 33, 19, 26, 35, 35, 33, 40, 23, 45, 42, 45, 34, 39, 51, 44, 49, 38, 29, 55, 31, 56, 62, 52, 55, 51, 46, 63, 69, 37, 70, 57, 66, 65, 75, 78, 41, 77, 68, 43, 58, 77, 85, 78, 69, 62, 47, 76, 93, 91

Offset: 1

Author

Cino Hilliard, Aug 12 2003

Keywords

• easy
• nonn

Examples

			For 91=7*13 we have 91-13 = 78.
For 92=2*2*23 we have 92-23 = 69.
		

Links

• Ivan Neretin, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A002808, A085271.

Programs

Mathematica

DeleteCases[Table[n - FactorInteger[n][[-1, 1]], {n, 100}], 0 ] (* _Ivan Neretin, Jun 20 2019 *)

PARI

cminusp2(n) = { for(x=2,n, forstep(p=x,2,-1, if(isprime(p) & x%p==0 & isprime(x)==0,print1(x-p,","); break); ) ) }

Extensions

Offset corrected by Ivan Neretin, Jun 20 2019