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A248835 a(n) = n + A033677(n), where A033677(n) is the smallest divisor of n >= sqrt(n). 2
2, 4, 6, 6, 10, 9, 14, 12, 12, 15, 22, 16, 26, 21, 20, 20, 34, 24, 38, 25, 28, 33, 46, 30, 30, 39, 36, 35, 58, 36, 62, 40, 44, 51, 42, 42, 74, 57, 52, 48, 82, 49, 86, 55, 54, 69, 94, 56, 56, 60, 68, 65, 106, 63, 66, 64, 76, 87, 118, 70, 122, 93 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Conjecture: For n > 1 there is at least one prime in [n, a(n)] exclusive.

a(n) = 2*n when n is prime.

When n = A002620(m), then a(n) = A002620(m+1), i.e., quarter-squares. Oppermann's conjecture states that there is at least one prime in [A002620(m), A002620(m+1)] exclusive.

When n is square, repeated values for a(n) occur at n-1 and n. These values are A002378(sqrt(n)), i.e., oblong numbers.

When n = A002378(m), then a(n) = (m+1)^2.

LINKS

Table of n, a(n) for n=1..62.

Wikipedia, Oppermann's Conjecture

EXAMPLE

When n = 40, the smallest divisor of 40 that is greater than or equal to sqrt(40) is 8 so a(40)=48.

MATHEMATICA

a248835[n_Integer] := n + Min[Select[Divisors[n], # >= Sqrt[n] &]]; a248835 /@ Range[120] (* Michael De Vlieger, Nov 10 2014 *)

PROG

(Sage)

[n+min([x for x in divisors(n) if x>=sqrt(n)]) for n in [1..100]] # Tom Edgar, Oct 15 2014

(PARI) a(n)=fordiv(n, d, if(d^2>=n, return(n+d))) \\ Charles R Greathouse IV, Oct 21 2014

CROSSREFS

Cf. A002378, A002620, A033677.

Sequence in context: A247653 A061228 A070229 * A053568 A037225 A060685

Adjacent sequences: A248832 A248833 A248834 * A248836 A248837 A248838

KEYWORD

nonn

AUTHOR

Bob Selcoe, Oct 15 2014

STATUS

approved

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Last modified November 26 21:15 EST 2022. Contains 358362 sequences. (Running on oeis4.)