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A257930 Primes equal to the sum of the prime factors, with multiplicity, of the previous k numbers, for some k. 4
23, 269, 1049, 1277, 8869951, 972928919, 74885169679 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Values of k are 2, 5, 4, 12, 12, 19, 37, ...

a(8) > 10^12. - Giovanni Resta, May 15 2015

Subsequence of prime terms of A257976. - Robert G. Wilson v, May 19 2015

LINKS

Table of n, a(n) for n=1..7.

EXAMPLE

For 23, consider the prime factors of the previous 2 numbers, 21, 22: 3, 7; 2, 11. Their sum is 3 + 7 + 2 + 11 = 23.

For 269, consider the prime factors of the previous 5 numbers, 264, 265, 266, 267, 268: 2, 2, 2, 3, 11; 5, 53; 2, 7, 19; 3, 89; 2, 2, 67. Their sum is 2 + 2 + 2 + 3 + 11 + 5 + 53 + 2 + 7 + 19 + 3 + 89 + 2 + 2 + 67 = 269.

MAPLE

with(numtheory): P:= proc(q) local a, d, j, k, n;

for n from 3 to q do if isprime(n) then a:=0; k:=0;

while a<n do k:=k+1; d:=ifactors(n-k)[2];

d:=add(d[j][1]*d[j][2], j=1..nops(d));

a:=a+d; od; if a=n then print(n);

fi; fi; od; end: P(10^9);

MATHEMATICA

sopfr[n_] := Plus @@ Times @@@ FactorInteger@ n; fQ[n_] := Block[{k = n - 1, s = 0}, While[s += sopfr@ k; s < n, k--]; s == n]; p = 5; lst = {}; While[p < 100000000, If[ fQ@ p, AppendTo[lst, p]]; p = NextPrime@ p]; lst (* Robert G. Wilson v, May 15 2015 *)

PROG

(PARI) sopfr(n)=my(f=factor(n)); sum(i=1, #f[, 1], f[i, 1]*f[i, 2]);

is(n)=if(n<23, return(0)); my(s); for(k=1, n, s+=sopfr(n-k); if(s>=n, return(n==s && isprime(n)))) \\ Charles R Greathouse IV, May 15 2015

CROSSREFS

Cf. A257367, A257524.

Sequence in context: A002681 A240876 A142220 * A142027 A161523 A161930

Adjacent sequences:  A257927 A257928 A257929 * A257931 A257932 A257933

KEYWORD

nonn,more

AUTHOR

Paolo P. Lava, May 13 2015

EXTENSIONS

a(6)-a(7) from Giovanni Resta, May 15 2015

STATUS

approved

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Last modified October 16 03:37 EDT 2019. Contains 328040 sequences. (Running on oeis4.)