cp's OEIS Frontend

This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.

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A076815 Initial indices of five successive primes squared with integer average.

Original entry on oeis.org

79, 258, 397, 428, 429, 502, 503, 609, 787, 788, 925, 926, 927, 1026, 1027, 1028, 1105, 1312, 1334, 1335, 1343, 1348, 1349, 1378, 1422, 1524, 1572, 1601, 1602, 1790, 1791, 1813, 2015, 2081, 2082, 2125, 2126, 2131, 2141, 2142, 2147, 2292, 2448, 2765, 2766
Offset: 1

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Author

Zak Seidov, Oct 17 2002

Keywords

Comments

Or, numbers n such that sum of 5 consecutive primes squared, starting with p(n), ends with 5.
Unlike the average of two, three, four and six successive primes squared (with initial indices > 1,2,1,2, respectively), the average of five successive primes squared is rarely an integer.
Cases of sums ending with 5 are much less numerous than cases with 1, 3, 7 and 9.
E.g. for the first 20000, sums with final digits 1, 3, 5, 7 and 9 are 7238, 2380, 466, 2529 and 7386 (and 1 case with final 8, 208=A131686(1)). And for first 200000 sums the corresponding numbers are 71166, 25820, 5956, 26075, 70982.
The explanation of this "deficiency of final 5's" is simple: assuming that final digits {1,3,7,9} of primes are equally often, we get that probabilities for final digits {1,3,5,7,9} of sum of squares of five primes are {10/32,5/32,2/32,5/32,10/32}.

Examples

			sum(prime(i)^2,i=79..83)/5=(401^2+409^2+419^2+421^2+431^2)/5=866645/5=173329=A076814(1),
sum(prime(i)^2,i=258..262)/5=(1627^2+1637^2+1657^2+1663^2+1667^2)/5=13617005/5=2723401=A076814(2).
		

Crossrefs

Programs

  • Mathematica
    PrimePi[Sqrt[#]]&/@Select[Partition[Prime[Range[3000]]^2,5,1],IntegerQ[ Mean[ #]]&][[All,1]] (* Harvey P. Dale, Jul 15 2017 *)

Extensions

Edited and merged with A131359 by Zak Seidov, May 18 2008 at the suggestion of R. J. Mathar
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