A290290 Numbers m having greatest prime power divisor d such that d is smaller than the difference between m and the largest prime smaller than m and d is smaller than the difference between m and twice the largest prime smaller than m/2.
3432, 5980, 12870, 12880, 13090, 14280, 14586, 20475, 28272, 28275, 31416, 31450, 34580, 35650, 39270, 45045, 45220, 72072, 76076, 96135, 97812, 106080, 106590, 120120, 121992, 125580, 132804, 139230, 173420, 181350, 185640, 191400, 195624, 202275, 203112, 215050, 216315, 222768, 232254, 240240, 266475
Offset: 1
Keywords
Examples
The first number of the sequence is 3432. 3432 = 2^3*3*11*13, so the greatest prime power divisor of 3432 is 13. The largest prime smaller than 3432 is 3413, and the difference between these two numbers is 19. Because 19 is greater than 13 the first property is not satisfied. 3432/2 = 1716, and the closest prime to 1716 is 1709. The difference between m and 2*1709 = 3418 is 14. 14 is also greater than 13, which means that the second property is also not satisfied. Thus 3432 is in the sequence.
Links
- Charles R Greathouse IV, Table of n, a(n) for n = 1..10000
- Sílvia Casacuberta Puig, On the divisibility of binomial coefficients, 2018; see also, arXiv:1906.07652 [math.NT], 2019.
- E. Kummer, Über die Ergänzungssätze zu den allgemeinen Reciprocitätsgesetzen, Journal für die reine und angewandte Mathematik, 44:93-146, 1852.
- E. Lucas, Théorie des fonctions numériques simplement périodiques, American Journal of Mathematics, 44:184-196, 1878.
- J. Shareshian and R. Woodroofe, Divisibility of binomial coefficients and generation of alternating groups, arXiv:1505.05143 [math.CO], 2015-2017.
- Wikipedia, Kummer's Theorem
- Wikipedia, Lucas' Theorem
Crossrefs
Cf. A034699.
Programs
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Mathematica
okQ[m_] := m > 2 && Module[{d = Max[Power @@@ FactorInteger[m]]}, d < m - NextPrime[m, -1] && d < m - 2 NextPrime[(m-1)/2, -1]]; Reap[For[m = 1, m <= 300000, m++, If[okQ[m], Print[m]; Sow[m]]]][[2, 1]] (* Jean-François Alcover, Feb 18 2019 *)
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PARI
a034699(n) = my(d = Vecrev(divisors(n))); for (k=1, #d, if (isprimepower(d[k]), return (d[k]));); isok(n) = my(thed = a034699(n)); (thed < (n - precprime(n-1))) && (thed < (n - 2*precprime((n-1)/2))); \\ Michel Marcus, Aug 08 2017
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PARI
is(n)=if(n<9, return(0)); my(f=factor(n),d=f[1,1]^f[1,2],t); for(i=2,#f~, t=f[i,1]^f[i,2]; if(t>d, d=t)); d < n-precprime(n-1) && d < n - 2*precprime((n-1)/2) \\ Charles R Greathouse IV, Aug 10 2017
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PARI
allUnitaryDivisorsLessThan(n,sz)=my(t,e); if(t==1, return(sz>1)); forprime(p=2,sz-1, e=valuation(n,p); t=p^e; if(t>1, if(t>=sz, return(0)); n/=t; if(n==1, return(1)))); 0 is(n)=n>9 && allUnitaryDivisorsLessThan(n, min(n - 2*precprime((n-1)/2), n-precprime(n-1))) \\ Charles R Greathouse IV, Aug 10 2017
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