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Search: id:A079143
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| A079143 |
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Numbers divisible by prime ceilings of their square roots + 1. |
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+0
1
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| 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; listen)
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OFFSET
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1,1
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COMMENT
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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...
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FORMULA
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a(n) = prime(ceiling(n/2))*(prime(ceiling(n/2)) - (n mod 2))
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EXAMPLE
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930 is in the sequence because ceiling(sqrt(930)) = 31 and 930/31 = 30.
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MATHEMATICA
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Flatten[ #(#-{1, 0})&/@Prime/@Range[30]]
a[n_] := (p=Prime[Ceiling[n/2]])(p-Mod[n, 2])
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PROGRAM
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(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.
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CROSSREFS
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Sequence in context: A110538 A098787 A164138 this_sequence A049912 A093081 A073659
Adjacent sequences: A079140 A079141 A079142 this_sequence A079144 A079145 A079146
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KEYWORD
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easy,nonn
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AUTHOR
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Cino Hilliard (hillcino368(AT)gmail.com), Dec 26 2002
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