login
The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A079143 Numbers divisible by prime ceilings of their square roots + 1. 1
2, 4, 6, 9, 20, 25, 42, 49, 110, 121, 156, 169, 272, 289, 342, 361, 506, 529, 812, 841, 930, 961, 1332, 1369, 1640, 1681, 1806, 1849, 2162, 2209, 2756, 2809, 3422, 3481, 3660, 3721, 4422, 4489, 4970, 5041, 5256, 5329, 6162, 6241, 6806, 6889, 7832, 7921 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

n is in the sequence if r=ceiling(sqrt(n)) is prime and r divides n.

Union of the 2 sequences A001248={p^2} and A036689={p(p-1)} for p prime.

Sum of the reciprocals = 1.225...

LINKS

Table of n, a(n) for n=1..48.

FORMULA

a(n) = prime(ceiling(n/2))*(prime(ceiling(n/2)) - (n mod 2))

EXAMPLE

930 is in the sequence because ceiling(sqrt(930)) = 31 and 930/31 = 30.

MATHEMATICA

Flatten[ #(#-{1, 0})&/@Prime/@Range[30]]

a[n_] := (p=Prime[Ceiling[n/2]])(p-Mod[n, 2])

PROG

(PARI) ipsqrt(n) = { sr= 0; for(x=1, n, v = ceil(sqrt(x)); if(isprime(v) && x%v == 0, print1(x" "); sr+=1.0/x; ); ); print(); print(sr); } \\ numbers divisible by the prime ceilings of their square roots.

CROSSREFS

Sequence in context: A320449 A263434 A192079 * A192316 A049912 A268868

Adjacent sequences:  A079140 A079141 A079142 * A079144 A079145 A079146

KEYWORD

easy,nonn

AUTHOR

Cino Hilliard, Dec 26 2002

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified June 6 17:28 EDT 2020. Contains 334830 sequences. (Running on oeis4.)