A304994 a(n) is the smallest number k such that sigma(k + n) = sigma(k) - n, or -1 if no such number exists.

1, 4, 6976, 8, 15

Offset: 0

Author

Michel Marcus, May 23 2018

Keywords

• nonn
• more

Comments

Further terms of the sequence: a(5) > 10^9 (if it exists), a(6) = 35, a(7) = 16, a(8) = 39, a(10) = 51, a(12) = 95, a(14) = 831, a(15) = 32, a(16) = 45, a(18) = 88, a(19) = 36, a(20) = 63, a(21) = 50, a(22) = 222, a(24) = 135, a(26) = 37888, a(28) = 96, a(30) = 284, a(32) = 117, a(34) = 354, a(36) = 175, a(37) = 1089, a(38) = 84, a(40) = 153, a(42) = 176, a(44) = 3893, a(46) = 267, a(48) = 210. - Daniel Suteu, May 25 2018

a(5) > 3*10^12, if it exists. - Giovanni Resta, May 26 2018

Examples

			sigma(4+1) - sigma(4) = -1, so a(1) = 4, since this does not happen for x < 4.
sigma(8+3) - sigma(8) = -3, so a(3) = 8, since this does not happen for x < 8.
		

Crossrefs

Cf. A000203, A015886.

Programs

PARI

a(n) = {my(k=1); while(sigma(k+n) != sigma(k) - n, k++); k;}