A111186 - OEIS

A111186

Difference between the closest squares surrounding squarefree composite numbers.

5, 5, 7, 7, 7, 7, 9, 9, 9, 9, 9, 11, 11, 11, 11, 11, 11, 11, 11, 13, 13, 13, 13, 13, 13, 13, 13, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 17, 17, 17, 17, 17, 17, 17, 17, 17, 17, 17, 17, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19

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OFFSET

6,1

LINKS

Table of n, a(n) for n=6..70.

FORMULA

Let n be a squarefree composite number and r = floor(sqrt(n)). Then the closest surrounding squares of n are r^2 and (r+1)^2. So d = (r+1)^2 - r^2 = 2r+1.

EXAMPLE

6 is the first positive squarefree composite number. 2^2 and 3^2 are the closest squares surrounding 6. So the difference, 9-4 = 5, is the first entry in the table.

PROG

(PARI) surrsq(n) = { local(x, y, j, r, d); for(x=1, n, if(!issquare(x)&!isprime(x), r=floor(sqrt(x)); d=r+r+1; print1(d", ") \ print1(r^2", "(r+1)^2", ") ) ) }

CROSSREFS

Sequence in context: A231589 A133888 A244587 * A127798 A053671 A028278

Adjacent sequences: A111183 A111184 A111185 * A111187 A111188 A111189

KEYWORD

easy,nonn

AUTHOR

Cino Hilliard, Nov 12 2005

STATUS

approved