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A244446 a(n) is the smallest integer m such that m-n is composite and phi(m-n) + sigma(m+n) = 2*m. 5
25, 323, 48, 34, 53, 471, 58, 78, 84, 76, 71, 122, 64, 144, 162, 118, 74, 188, 106, 258, 156, 2512, 68, 254, 94, 107, 132, 2326, 876, 536, 154, 182, 268, 468, 98, 2061, 106, 408, 264, 286, 258, 1520900, 423, 618, 276, 648, 579, 518, 204, 708, 196, 370, 164, 1088, 300, 1518, 412, 3616, 158, 1226 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

For each n, a(n)>n and like a(n)-n, a(n)+n is also composite.

If both numbers p & p+2n are primes the x=p+n is a solution for the equation phi(x-n)+sigma(x+n)=2x. But for these many solutions x, both x-n & x+n are primes.

LINKS

Table of n, a(n) for n=1..60.

EXAMPLE

a(1)=25 because 25-1 is composite, phi(25-1)+sigma(25+1)=2*25 and there is no such number less than 25.

MATHEMATICA

a[n_]:=(For[m=n+1, PrimeQ[m-n]||EulerPhi[m-n]+DivisorSigma[1, m+n]!=2m, m++]; m); Table[a[n], {n, 60}]

PROG

(PARI)

a(n)=m=n+4; while(isprime(m-n)||eulerphi(m-n)+sigma(m+n)!=2*m, m++); m

vector(100, n, a(n)) \\ Derek Orr, Aug 30 2014

CROSSREFS

Cf. A000010, A000203, A244447, A244448.

Sequence in context: A125437 A250318 A250559 * A246625 A161525 A161932

Adjacent sequences:  A244443 A244444 A244445 * A244447 A244448 A244449

KEYWORD

nonn

AUTHOR

Jahangeer Kholdi and Farideh Firoozbakht, Aug 30 2014

STATUS

approved

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Last modified April 25 12:04 EDT 2019. Contains 322456 sequences. (Running on oeis4.)