

A117825


Distance from nth highly composite number (cf. A002182) to nearest prime.


9



1, 0, 1, 1, 1, 1, 1, 1, 1, 7, 1, 1, 1, 1, 1, 1, 11, 1, 1, 1, 1, 1, 1, 11, 13, 1, 11, 1, 17, 1, 1, 13, 13, 1, 1, 17, 1, 17, 1, 1, 17, 17, 17, 1, 1, 19, 37, 37, 1, 17, 23, 1, 29, 1, 1, 19, 1, 19, 23, 1, 19, 31, 1, 19, 1, 1, 1, 1, 23, 1, 29, 23, 23, 1, 23, 71, 37, 1, 1, 31, 1, 23, 53, 1, 31
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OFFSET

1,10


COMMENTS

a) Conjecture: entries > 1 will always be prime. The entry will be larger than the largest prime factor of the highly composite number.
b) Will 1 always be the most common entry?
c) While a prime may always be located close to each highly composite number, is the converse false?
d) Is there always a prime between successive highly composite numbers?
From Antti Karttunen, Feb 26 2019: (Start)
The second sentence of point (a) follows as both gcd(n, A151799(n)) = 1 and gcd(A151800(n), n) = 1 for all n > 2 and the fact that the highly composite numbers are products of primorials, A002110 (with the least coprime prime > the largest prime factor). See also the conjectures and notes in A129912 and A141345. (End)


LINKS

Charles R Greathouse IV, Table of n, a(n) for n = 1..19999
Bill McEachen, Alternate plot, Wikimedia Commons.
Graeme McRae, Highly Composite Numbers.
Wikipedia, Highly Composite Numbers.
Wikipedia, Divisor Function (sigma).


FORMULA

a(1) = 1; for n > 1, a(n) = min(A141345(n), A324385(n)).  Antti Karttunen, Feb 26 2019


EXAMPLE

a(5) = abs(1211) = 1.


MATHEMATICA

With[{s = DivisorSigma[0, Range[Product[Prime@ i, {i, 8}]]]}, Map[If[PrimeQ@ #, 0, Min[#  NextPrime[#, 1], NextPrime[#]  #]] &@ FirstPosition[s, #][[1]] &, Union@ FoldList[Max, s]]] (* Michael De Vlieger, Mar 11 2019 *)


PROG

(PARI)
A141345(n) = (nextprime(1+A002182(n))A002182(n));
A324385(n) = (A002182(n)precprime(A002182(n)));
A117825(n) = if(1==n, 1, min(A141345(n), A324385(n))); \\ Antti Karttunen, Feb 26 2019


CROSSREFS

Cf. A002100, A002182, A007917, A129912, A141345, A151800, A324385.
Sequences tied to conjecture a): A228943, A228945.
Cf. also A005235, A060270.
Sequence in context: A081229 A109010 A268354 * A010143 A101027 A129408
Adjacent sequences: A117822 A117823 A117824 * A117826 A117827 A117828


KEYWORD

nonn,look


AUTHOR

Bill McEachen, May 01 2006


EXTENSIONS

More terms from Don Reble, May 02 2006


STATUS

approved



