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A056139
a(n) = n^2 - primefloor(n)*primeceiling(n).
3
0, 0, 1, 0, 1, 0, -13, 4, 23, 0, 1, 0, -25, 4, 35, 0, 1, 0, -37, 4, 47, 0, -91, -42, 9, 62, 117, 0, 1, 0, -123, -58, 9, 78, 149, 0, -73, 4, 83, 0, 1, 0, -85, 4, 95, 0, -187, -90, 9, 110, 213, 0, -211, -102, 9, 122, 237, 0, 1, 0, -243, -118, 9, 138, 269, 0, -133, 4, 143, 0, 1, 0, -291, -142, 9, 162, 317, 0, -157, 4, 167, 0, -331, -162, 9
OFFSET
2,7
COMMENTS
a(n)= 0 iff n is prime.
LINKS
FORMULA
a(n) = n^2 - A007917(n)*A007918(n) = A000290(n) - A030664(n).
EXAMPLE
a(3)=3^2-3*3=0, a(4)=4^2-3*5=1
PROG
(PARI)
A030664(n) = if (n < 2, 1, precprime(n)*nextprime(n)); \\ From A030664
A056139(n) = (n^2 - A030664(n)); \\ Antti Karttunen, Mar 20 2018
CROSSREFS
KEYWORD
easy,sign
AUTHOR
Henry Bottomley, Jun 15 2000
EXTENSIONS
More terms from Antti Karttunen, Mar 20 2018
STATUS
approved